diff --git a/src/cmd/dist/util.go b/src/cmd/dist/util.go
index 0a419e465fe..e99375f5380 100644
--- a/src/cmd/dist/util.go
+++ b/src/cmd/dist/util.go
@@ -389,6 +389,10 @@ func xgetgoarm() string {
 		// sense to auto-detect the setting.
 		return "7"
 	}
+	if goos == "windows" {
+		// windows/arm only works with ARMv7 executables.
+		return "7"
+	}
 	if gohostarch != "arm" || goos != gohostos {
 		// Conservative default for cross-compilation.
 		return "5"
diff --git a/src/cmd/go/internal/mvs/mvs.go b/src/cmd/go/internal/mvs/mvs.go
index b630b610f10..bed4d5c1ba5 100644
--- a/src/cmd/go/internal/mvs/mvs.go
+++ b/src/cmd/go/internal/mvs/mvs.go
@@ -293,10 +293,15 @@ func Req(target module.Version, base []string, reqs Reqs) ([]module.Version, err
 	}
 	// First walk the base modules that must be listed.
 	var min []module.Version
+	haveBase := map[string]bool{}
 	for _, path := range base {
+		if haveBase[path] {
+			continue
+		}
 		m := module.Version{Path: path, Version: max[path]}
 		min = append(min, m)
 		walk(m)
+		haveBase[path] = true
 	}
 	// Now the reverse postorder to bring in anything else.
 	for i := len(postorder) - 1; i >= 0; i-- {
@@ -370,10 +375,19 @@ func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]mod
 // reqs.Previous, but the methods of reqs must otherwise handle such versions
 // correctly.
 func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([]module.Version, error) {
-	list, err := reqs.Required(target)
+	// Per https://research.swtch.com/vgo-mvs#algorithm_4:
+	// “To avoid an unnecessary downgrade to E 1.1, we must also add a new
+	// requirement on E 1.2. We can apply Algorithm R to find the minimal set of
+	// new requirements to write to go.mod.”
+	//
+	// In order to generate those new requirements, we need to identify versions
+	// for every module in the build list — not just reqs.Required(target).
+	list, err := BuildList(target, reqs)
 	if err != nil {
 		return nil, err
 	}
+	list = list[1:] // remove target
+
 	max := make(map[string]string)
 	for _, r := range list {
 		max[r.Path] = r.Version
@@ -406,6 +420,9 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 		}
 		added[m] = true
 		if v, ok := max[m.Path]; ok && reqs.Max(m.Version, v) != v {
+			// m would upgrade an existing dependency — it is not a strict downgrade,
+			// and because it was already present as a dependency, it could affect the
+			// behavior of other relevant packages.
 			exclude(m)
 			return
 		}
@@ -422,6 +439,7 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 			// is transient (we couldn't download go.mod), return the error from
 			// Downgrade. Currently, we can't tell what kind of error it is.
 			exclude(m)
+			return
 		}
 		for _, r := range list {
 			add(r)
@@ -433,8 +451,8 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 		}
 	}
 
-	var out []module.Version
-	out = append(out, target)
+	downgraded := make([]module.Version, 0, len(list)+1)
+	downgraded = append(downgraded, target)
 List:
 	for _, r := range list {
 		add(r)
@@ -461,10 +479,14 @@ List:
 			add(p)
 			r = p
 		}
-		out = append(out, r)
+		downgraded = append(downgraded, r)
 	}
 
-	return out, nil
+	return BuildList(target, &override{
+		target: target,
+		list:   downgraded,
+		Reqs:   reqs,
+	})
 }
 
 type override struct {
diff --git a/src/cmd/go/internal/mvs/mvs_test.go b/src/cmd/go/internal/mvs/mvs_test.go
index 721cd9635c8..742e396e0dc 100644
--- a/src/cmd/go/internal/mvs/mvs_test.go
+++ b/src/cmd/go/internal/mvs/mvs_test.go
@@ -28,10 +28,11 @@ D4: E2 F1
 D5: E2
 G1: C4
 A2: B1 C4 D4
-build A: A B1 C2 D4 E2 F1
-upgrade* A: A B1 C4 D5 E2 F1 G1
-upgrade A C4: A B1 C4 D4 E2 F1 G1
-downgrade A2 D2: A2 C4 D2
+build A:       A B1 C2 D4 E2 F1
+upgrade* A:    A B1 C4 D5 E2 F1 G1
+upgrade A C4:  A B1 C4 D4 E2 F1 G1
+build A2:     A2 B1 C4 D4 E2 F1 G1
+downgrade A2 D2: A2 C4 D2 E2 F1 G1
 
 name: trim
 A: B1 C2
@@ -68,7 +69,7 @@ B2: D1
 C: D2
 D1: E2
 D2: E1
-build A: A B1 C D2 E1
+build A:      A B1 C D2 E1
 upgrade A B2: A B2 C D2 E2
 
 name: cross1R
@@ -136,17 +137,17 @@ name: cross5
 A: D1
 D1: E2
 D2: E1
-build A: A D1 E2
-upgrade* A: A D2 E2
-upgrade A D2: A D2 E2
+build A:       A D1 E2
+upgrade* A:    A D2 E2
+upgrade A D2:  A D2 E2
 upgradereq A D2: D2 E2
 
 name: cross6
 A: D2
 D1: E2
 D2: E1
-build A: A D2 E1
-upgrade* A: A D2 E2
+build A:      A D2 E1
+upgrade* A:   A D2 E2
 upgrade A E2: A D2 E2
 
 name: cross7
@@ -175,7 +176,7 @@ B1: D1
 B2:
 C2:
 D2:
-build A: A B1 C1 D1
+build A:    A B1 C1 D1
 upgrade* A: A B2 C2 D2
 
 name: simplify
@@ -194,7 +195,7 @@ B4:
 B5.hidden:
 C2:
 C3:
-build A: A B1 C1
+build A:    A B1 C1
 upgrade* A: A B4 C3
 
 name: up2
@@ -206,15 +207,15 @@ B4:
 B5.hidden:
 C2:
 C3:
-build A: A B5.hidden C1
+build A:    A B5.hidden C1
 upgrade* A: A B5.hidden C3
 
 name: down1
 A: B2
 B1: C1
 B2: C2
-build A: A B2 C2
-downgrade A C1: A B1
+build A:        A B2 C2
+downgrade A C1: A B1 C1
 
 name: down2
 A: B2 E2
@@ -227,12 +228,51 @@ D2: B2
 E2: D2
 E1:
 F1:
-downgrade A F1: A B1 E1
+build A:        A B2 C2 D2 E2 F2
+downgrade A F1: A B1 C1 D1 E1 F1
+
+# https://research.swtch.com/vgo-mvs#algorithm_4:
+# “[D]owngrades are constrained to only downgrade packages, not also upgrade
+# them; if an upgrade before downgrade is needed, the user must ask for it
+# explicitly.”
+#
+# Here, downgrading B2 to B1 upgrades C1 to C2, and C2 does not depend on D2.
+# However, C2 would be an upgrade — not a downgrade — so B1 must also be
+# rejected.
+name: downcross1
+A: B2 C1
+B1: C2
+B2: C1
+C1: D2
+C2:
+D1:
+D2:
+build A:        A B2 C1 D2
+downgrade A D1: A       D1
+
+# https://research.swtch.com/vgo-mvs#algorithm_4:
+# “Unlike upgrades, downgrades must work by removing requirements, not adding
+# them.”
+#
+# However, downgrading a requirement may introduce a new requirement on a
+# previously-unrequired module. If each dependency's requirements are complete
+# (“tidy”), that can't change the behavior of any other package whose version is
+# not also being downgraded, so we should allow it.
+name: downcross2
+A: B2
+B1: C1
+B2: D2
+C1:
+D1:
+D2:
+build A:        A B2    D2
+downgrade A D1: A B1 C1 D1
 
 name: downcycle
 A: A B2
 B2: A
 B1:
+build A:        A B2
 downgrade A B1: A B1
 
 # golang.org/issue/25542.
@@ -240,6 +280,7 @@ name: noprev1
 A: B4 C2
 B2.hidden:
 C2:
+build A:               A B4        C2
 downgrade A B2.hidden: A B2.hidden C2
 
 name: noprev2
@@ -247,6 +288,7 @@ A: B4 C2
 B2.hidden:
 B1:
 C2:
+build A:               A B4        C2
 downgrade A B2.hidden: A B2.hidden C2
 
 name: noprev3
@@ -254,6 +296,7 @@ A: B4 C2
 B3:
 B2.hidden:
 C2:
+build A:               A B4        C2
 downgrade A B2.hidden: A B2.hidden C2
 
 # Cycles involving the target.
@@ -264,9 +307,9 @@ A: B1
 B1: A1
 B2: A2
 B3: A3
-build A: A B1
+build A:      A B1
 upgrade A B2: A B2
-upgrade* A: A B3
+upgrade* A:   A B3
 
 # golang.org/issue/29773:
 # Requirements of older versions of the target
@@ -280,7 +323,7 @@ B2: A2
 C1: A2
 C2:
 D2:
-build A: A B1 C1 D1
+build A:    A B1 C1 D1
 upgrade* A: A B2 C2 D2
 
 # Cycles with multiple possible solutions.
@@ -293,23 +336,23 @@ B2: C2
 C1:
 C2: B2
 build M: M A1 B2 C2
-req M: A1 B2
-req M A: A1 B2
-req M C: A1 C2
+req M:     A1 B2
+req M A:   A1 B2
+req M C:   A1 C2
 
 # Requirement minimization.
 
 name: req1
 A: B1 C1 D1 E1 F1
 B1: C1 E1 F1
-req A: B1 D1
+req A:   B1    D1
 req A C: B1 C1 D1
 
 name: req2
 A: G1 H1
 G1: H1
 H1: G1
-req A: G1
+req A:   G1
 req A G: G1
 req A H: H1
 
@@ -326,7 +369,20 @@ M: Anone B1 D1 E1
 B1: Cnone D1
 E1: Fnone
 build M: M B1 D1 E1
-req M: B1 E1
+req M:     B1    E1
+
+name: reqdup
+M: A1 B1
+A1: B1
+B1:
+req M A A: A1
+
+name: reqcross
+M: A1 B1 C1
+A1: B1 C1
+B1: C1
+C1:
+req M A B: A1 B1
 `
 
 func Test(t *testing.T) {
diff --git a/src/cmd/go/testdata/script/mod_get_downadd_indirect.txt b/src/cmd/go/testdata/script/mod_get_downadd_indirect.txt
new file mode 100644
index 00000000000..efc38f77c85
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downadd_indirect.txt
@@ -0,0 +1,81 @@
+# This test illustrates a case where downgrading one module may upgrade another.
+# Compare to the downcross2 test case in cmd/go/internal/mvs/mvs_test.go.
+
+# The initial package import graph used in this test looks like:
+#
+# a ---- b ---- d
+#
+# The module dependency graph originally looks like:
+#
+# a ---- b.2 ---- d.2
+#
+# b.1 ---- c.1
+#
+# If we downgrade module d to version 1, we must downgrade b as well.
+# If that downgrade selects b version 1, we will add a new dependency on module c.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go get -d example.com/d@v0.1.0
+go list -m all
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.1.0 '
+
+-- go.mod --
+module example.com/a
+
+go 1.15
+
+require example.com/b v0.2.0
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- a.go --
+package a
+
+import _ "example.com/b"
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/d v0.2.0
+-- b2/b.go --
+package b
+
+import _ "example.com/d"
+
+-- c/go.mod --
+module example.com/c
+
+go 1.15
+
+-- c/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.15
+-- d/d.go --
+package d
diff --git a/src/cmd/go/testdata/script/mod_get_downup_artifact.txt b/src/cmd/go/testdata/script/mod_get_downup_artifact.txt
new file mode 100644
index 00000000000..b35d4c4fd03
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downup_artifact.txt
@@ -0,0 +1,165 @@
+# This test illustrates a case where an upgrade–downgrade–upgrade cycle can
+# result in upgrades of otherwise-irrelevant dependencies.
+#
+# This case has no corresponding test in the mvs package, because it is an
+# artifact that results from the composition of *multiple* MVS operations.
+
+# The initial package import graph used in the test looks like:
+#
+# m ---- a
+# |      |
+# +----- b
+# |      |
+# +----- c
+# |
+# +----- d
+#
+# b version 2 adds its own import of package d.
+#
+# The module dependency graph initially looks like:
+#
+# m ---- a.1
+# |       |
+# +----- b.1
+# |       |
+# +----- c.1
+# |
+# +----- d.1
+#
+# b.2 ---- c.2
+#  |
+#  +------ d.2
+#  |
+#  +------ e.1
+#
+# If we upgrade module b to version 2, we will upgrade c and d and add a new
+# dependency on e. If b version 2 is disallowed because of any of those
+# dependencies, the other dependencies should not be upgraded as a side-effect.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.1.0 '
+! stdout '^example.com/e '
+
+# b is imported by a, so the -u flag would normally upgrade it to v0.2.0.
+# However, that would conflict with the explicit c@v0.1.0 constraint,
+# so b must remain at v0.1.0.
+#
+# If we're not careful, we might temporarily add b@v0.2.0 and pull in its
+# upgrades of module d and addition of module e, which are not relevant to
+# b@v0.1.0 and should not be added to the main module's dependencies.
+
+go get -u -d example.com/a@latest example.com/c@v0.1.0
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+
+	# BUG: d should remain at v0.1.0, because it is not transitively imported by a
+	# with b@v0.1.0. Today, it is spuriously upgraded to v0.2.0.
+stdout '^example.com/d v0.2.0 '
+
+	# BUG: e should not be added, because it is not transitively imported by a
+	# with b@v0.1.0. Today, it is spuriously added.
+stdout '^example.com/e v0.1.0 '
+
+-- go.mod --
+module example.com/m
+
+go 1.16
+
+require (
+	example.com/a v0.1.0
+	example.com/b v0.1.0
+	example.com/c v0.1.0
+	example.com/d v0.1.0
+)
+
+replace (
+	example.com/a v0.1.0 => ./a1
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c
+	example.com/c v0.2.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+	example.com/e v0.1.0 => ./e
+)
+-- m.go --
+package m
+
+import (
+	_ "example.com/a"
+	_ "example.com/b"
+	_ "example.com/c"
+	_ "example.com/d"
+)
+
+-- a1/go.mod --
+module example.com/a
+
+go 1.16
+
+require example.com/b v0.1.0
+-- a1/a.go --
+package a
+
+import _ "example.com/b"
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.16
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.16
+
+require (
+	example.com/c v0.2.0
+	example.com/d v0.2.0
+	example.com/e v0.1.0
+)
+-- b2/b.go --
+package b
+
+import (
+	"example.com/c"
+	"example.com/d"
+	"example.com/e"
+)
+
+-- c/go.mod --
+module example.com/c
+
+go 1.16
+-- c/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.16
+-- d/d.go --
+package d
+
+-- e/go.mod --
+module example.com/e
+
+go 1.16
+-- e/e.go --
+package e
diff --git a/src/cmd/go/testdata/script/mod_get_downup_indirect.txt b/src/cmd/go/testdata/script/mod_get_downup_indirect.txt
new file mode 100644
index 00000000000..ced1dcd6b14
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downup_indirect.txt
@@ -0,0 +1,101 @@
+# This test illustrates a case where downgrading one module may upgrade another.
+# Compare to the downcross1 test case in cmd/go/internal/mvs/mvs_test.go.
+
+# The package import graph used in this test looks like:
+#
+# a ---- b
+#   \     \
+#    \     \
+#     ----- c ---- d
+#
+# The module dependency graph originally looks like:
+#
+# a ---- b.2
+#   \      \
+#    \      \
+#     ----- c.1 ---- d.2
+#
+# b.1 ---- c.2
+#
+# If we downgrade module d to version 1, we must downgrade b as well.
+# If that downgrade selects b version 1, we will upgrade module c to version 2.
+# So 'go get d@1' should instead downgrade both b and c to "none".
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go get -d example.com/d@v0.1.0
+go list -m all
+! stdout '^example.com/b '
+! stdout '^example.com/c '
+stdout '^example.com/d v0.1.0 '
+
+-- go.mod --
+module example.com/a
+
+go 1.15
+
+require (
+	example.com/b v0.2.0
+	example.com/c v0.1.0
+)
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- a.go --
+package a
+
+import (
+	_ "example.com/b"
+	_ "example.com/c"
+)
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.2.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b2/b.go --
+package b
+
+import _ "example.com/c"
+
+-- c1/go.mod --
+module example.com/c
+
+go 1.15
+
+require example.com/d v0.2.0
+-- c1/c.go --
+package c
+
+-- c2/go.mod --
+module example.com/c
+
+go 1.15
+-- c2/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.15
diff --git a/src/cmd/go/testdata/script/mod_load_badchain.txt b/src/cmd/go/testdata/script/mod_load_badchain.txt
index c0c382bfa63..32d9fb24d11 100644
--- a/src/cmd/go/testdata/script/mod_load_badchain.txt
+++ b/src/cmd/go/testdata/script/mod_load_badchain.txt
@@ -74,8 +74,7 @@ go get: example.com/badchain/c@v1.0.0 updating to
 	module declares its path as: badchain.example.com/c
 	        but was required as: example.com/badchain/c
 -- update-a-expected --
-go get: example.com/badchain/a@v1.0.0 updating to
-	example.com/badchain/a@v1.1.0 requires
+go get: example.com/badchain/a@v1.1.0 requires
 	example.com/badchain/b@v1.1.0 requires
 	example.com/badchain/c@v1.1.0: parsing go.mod:
 	module declares its path as: badchain.example.com/c
diff --git a/src/runtime/asm_riscv64.s b/src/runtime/asm_riscv64.s
index 31e324d6775..3d0349471a2 100644
--- a/src/runtime/asm_riscv64.s
+++ b/src/runtime/asm_riscv64.s
@@ -449,7 +449,7 @@ TEXT callRet<>(SB), NOSPLIT, $40-0
 	MOV	A1, 16(X2)
 	MOV	A3, 24(X2)
 	MOV	A2, 32(X2)
-	MOV	$0, 40(X2)
+	MOV	ZERO, 40(X2)
 	CALL	runtime·reflectcallmove(SB)
 	RET
 
diff --git a/src/runtime/cgo/linux_syscall.c b/src/runtime/cgo/linux_syscall.c
index 56f3d67d8bf..59761c8b409 100644
--- a/src/runtime/cgo/linux_syscall.c
+++ b/src/runtime/cgo/linux_syscall.c
@@ -32,7 +32,7 @@ typedef struct {
 
 #define SET_RETVAL(fn) \
   uintptr_t ret = (uintptr_t) fn ; \
-  if (ret == -1) {                 \
+  if (ret == (uintptr_t) -1) {	   \
     x->retval = (uintptr_t) errno; \
   } else                           \
     x->retval = ret