Remove ring0 floating point save/load functions on amd64.

ring0.Save/LoadFloatingPoint() are only usable if the caller can ensure that Go
will not clobber floating point registers before/after calling them
respectively. Due to regabig in Go 1.17, this is no longer the case; regabig
(among other things) maintains a zeroed XMM15 during ABIInternal execution,
including by zeroing it after ABI0-to-ABIInternal transitions. In
ring0.sysenter/exception, this happens in
ring0.kernelSyscall/kernelException.abi0 respectively; in
ring0.CPU.SwitchToUser, this happens after returning from
ring0.sysret/iret.abi0. Delete these functions and do floating point save/load
in assembly.

While arm64 doesn't appear to be immediately affected (so this CL permits us to
resume usage of Go 1.17), its use of Save/LoadFloatingPoint() still seems to be
incorrect for the same fundamental reason (Go code can't sanely assume what
registers the Go compiler will or won't use) and should be fixed eventually.

PiperOrigin-RevId: 401895658

1 files changed, 2 insertions, 40 deletions

diff --git a/pkg/ring0/lib_amd64.go b/pkg/ring0/lib_amd64.go
index 05c394ff5..c42a5b205 100644
--- a/pkg/ring0/lib_amd64.go
+++ b/pkg/ring0/lib_amd64.go
@@ -21,29 +21,6 @@ import (
 	"gvisor.dev/gvisor/pkg/cpuid"
 )
 
-// LoadFloatingPoint loads floating point state by the most efficient mechanism
-// available (set by Init).
-var LoadFloatingPoint func(*byte)
-
-// SaveFloatingPoint saves floating point state by the most efficient mechanism
-// available (set by Init).
-var SaveFloatingPoint func(*byte)
-
-// fxrstor uses fxrstor64 to load floating point state.
-func fxrstor(*byte)
-
-// xrstor uses xrstor to load floating point state.
-func xrstor(*byte)
-
-// fxsave uses fxsave64 to save floating point state.
-func fxsave(*byte)
-
-// xsave uses xsave to save floating point state.
-func xsave(*byte)
-
-// xsaveopt uses xsaveopt to save floating point state.
-func xsaveopt(*byte)
-
 // writeFS sets the FS base address (selects one of wrfsbase or wrfsmsr).
 func writeFS(addr uintptr)
 
@@ -53,8 +30,8 @@ func wrfsbase(addr uintptr)
 // wrfsmsr writes to the GS_BASE MSR.
 func wrfsmsr(addr uintptr)
 
-// WriteGS sets the GS address (set by init).
-var WriteGS func(addr uintptr)
+// writeGS sets the GS address (selects one of wrgsbase or wrgsmsr).
+func writeGS(addr uintptr)
 
 // wrgsbase writes to the GS base address.
 func wrgsbase(addr uintptr)
@@ -106,19 +83,4 @@ func Init(featureSet *cpuid.FeatureSet) {
 	hasXSAVE = featureSet.UseXsave()
 	hasFSGSBASE = featureSet.HasFeature(cpuid.X86FeatureFSGSBase)
 	validXCR0Mask = uintptr(featureSet.ValidXCR0Mask())
-	if hasXSAVEOPT {
-		SaveFloatingPoint = xsaveopt
-		LoadFloatingPoint = xrstor
-	} else if hasXSAVE {
-		SaveFloatingPoint = xsave
-		LoadFloatingPoint = xrstor
-	} else {
-		SaveFloatingPoint = fxsave
-		LoadFloatingPoint = fxrstor
-	}
-	if hasFSGSBASE {
-		WriteGS = wrgsbase
-	} else {
-		WriteGS = wrgsmsr
-	}
 }