Go 1.17 support for the KVM platform

Go 1.17 adds a new register-based calling convention. While transparent for
most applications, the KVM platform needs special work in a few cases.

First of all, we need the actual address of some assembly functions, rather
than the address of a wrapper. See http://gvisor.dev/pr/5832 for complete
discussion of this.

More relevant to this CL is that ABI0-to-ABIInternal wrappers (i.e., calls from
assembly to Go) access the G via FS_BASE. The KVM quite fast-and-loose about
the Go environment, often calling into (nosplit) Go functions with
uninitialized FS_BASE.

That will no longer work in Go 1.17, so this CL changes the platform to
consistently restore FS_BASE before calling into Go code.

This CL does not affect arm64 code. Go 1.17 does not support the register-based
calling convention for arm64 (it will come in 1.18), but arm64 also does not
use a non-standard register like FS_BASE for TLS, so it may not require any
changes.

PiperOrigin-RevId: 384234305

1 files changed, 2 insertions, 4 deletions

diff --git a/pkg/ring0/lib_amd64.go b/pkg/ring0/lib_amd64.go
index 3e6bb9663..46746fd80 100644
--- a/pkg/ring0/lib_amd64.go
+++ b/pkg/ring0/lib_amd64.go
@@ -43,8 +43,8 @@ func xsave(*byte)
 // xsaveopt uses xsaveopt to save floating point state.
 func xsaveopt(*byte)
 
-// WriteFS sets the GS address (set by init).
-var WriteFS func(addr uintptr)
+// writeFS sets the FS base address (selects one of wrfsbase or wrfsmsr).
+func writeFS(addr uintptr)
 
 // wrfsbase writes to the GS base address.
 func wrfsbase(addr uintptr)
@@ -116,10 +116,8 @@ func Init(featureSet *cpuid.FeatureSet) {
 		LoadFloatingPoint = fxrstor
 	}
 	if hasFSGSBASE {
-		WriteFS = wrfsbase
 		WriteGS = wrgsbase
 	} else {
-		WriteFS = wrfsmsr
 		WriteGS = wrgsmsr
 	}
 }