systrap: Enable using xsaveopt.

PiperOrigin-RevId: 554906814

pkg/sentry/platform/systrap/shared_context.go

@@ -172,16 +172,12 @@ func (sc *sharedContext) threadID() uint32 {
 
 // EnableSentryFastPath indicates that the polling mode is enabled for the
 // Sentry. It has to be called before putting the context into the context queue.
-// This function is used if contextDecouplingExp=true because the fastpath
-// is negotiated in ThreadContext.
 func (sc *sharedContext) enableSentryFastPath() {
 	atomic.StoreUint32(&sc.shared.SentryFastPath, 1)
 }
 
 // DisableSentryFastPath indicates that the polling mode for the sentry is
 // disabled for the Sentry.
-// This function is used if contextDecouplingExp=true because the fastpath
-// is negotiated in ThreadContext.
 func (sc *sharedContext) disableSentryFastPath() {
 	atomic.StoreUint32(&sc.shared.SentryFastPath, 0)
 }

pkg/sentry/platform/systrap/sysmsg/syshandler_amd64.S

@@ -125,17 +125,11 @@ restored_gs:
   movl $0xffffffff, %eax
   movl $0xffffffff, %edx
   movl __export_arch_state+offsetof_arch_state_xsave_mode(%rip), %esi
-  // TODO(b/268366549): Fix use of xsavec/xsaveopt.
-  // cmpl $XSAVE_MODE_XSAVEC, %esi
-  //   jl use_xsaveopt
-  //   xsavec (%rdi)
-  //   jmp fpu_saved
-  // use_xsaveopt:
-  //   cmpl $XSAVE_MODE_XSAVEOPT, %esi
-  //   jl use_xsave
-  //   xsaveopt (%rdi)
-  //   jmp fpu_saved
-  // use_xsave:
+  cmpl $XSAVE_MODE_XSAVEOPT, %esi
+  jl use_xsave
+  xsaveopt (%rdi)
+  jmp fpu_saved
+use_xsave:
   cmpl $XSAVE_MODE_XSAVE, %esi
   jl use_fxsave
   xsave (%rdi)

pkg/sentry/platform/systrap/sysmsg/sysmsg_amd64.go

@@ -38,11 +38,13 @@ type ArchState struct {
 	fsgsbase  uint32
 }
 
+// The linux kernel does not allow using xsavec from userspace, so we are limited
+// to xsaveopt.
+// See arch/x86/kernel/fpu/xstate.c:validate_user_xstate_header for details.
 const (
 	fxsave = iota
 	xsave
 	xsaveopt
-	xsavec
 )
 
 // Init initializes the arch specific state.
@@ -51,8 +53,9 @@ func (s *ArchState) Init() {
 
 	fpLenUint, _ := fs.ExtendedStateSize()
 	s.fpLen = uint32(fpLenUint)
-	// TODO(b/268366549): Fix use of xsavec/xsaveopt.
-	if fs.UseXsave() {
+	if fs.UseXsaveopt() {
+		s.xsaveMode = xsaveopt
+	} else if fs.UseXsave() {
 		s.xsaveMode = xsave
 	} else {
 		s.xsaveMode = fxsave

pkg/sentry/platform/systrap/sysmsg/sysmsg_offsets_amd64.h

@@ -24,7 +24,6 @@
 #define XSAVE_MODE_FXSAVE (0x0)
 #define XSAVE_MODE_XSAVE (0x1)
 #define XSAVE_MODE_XSAVEOPT (0x2)
-#define XSAVE_MODE_XSAVEC (0x3)
 
 // LINT.ThenChange(sysmsg.h, sysmsg_amd64.go)
 // LINT.IfChange