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

5 files changed, 6 insertions, 31 deletions

diff --git a/pkg/sentry/platform/kvm/bluepill_amd64.go b/pkg/sentry/platform/kvm/bluepill_amd64.go
index 0567c8d32..b2db2bb9f 100644
--- a/pkg/sentry/platform/kvm/bluepill_amd64.go
+++ b/pkg/sentry/platform/kvm/bluepill_amd64.go
@@ -71,10 +71,6 @@ func (c *vCPU) KernelSyscall() {
 	if regs.Rax != ^uint64(0) {
 		regs.Rip -= 2 // Rewind.
 	}
-	// We only trigger a bluepill entry in the bluepill function, and can
-	// therefore be guaranteed that there is no floating point state to be
-	// loaded on resuming from halt. We only worry about saving on exit.
-	ring0.SaveFloatingPoint(c.floatingPointState.BytePointer()) // escapes: no.
 	// N.B. Since KernelSyscall is called when the kernel makes a syscall,
 	// FS_BASE is already set for correct execution of this function.
 	//
@@ -112,8 +108,6 @@ func (c *vCPU) KernelException(vector ring0.Vector) {
 		regs.Rip = 0
 	}
 	// See above.
-	ring0.SaveFloatingPoint(c.floatingPointState.BytePointer()) // escapes: no.
-	// See above.
 	ring0.HaltAndWriteFSBase(regs) // escapes: no, reload host segment.
 }
 
@@ -144,5 +138,5 @@ func bluepillArchExit(c *vCPU, context *arch.SignalContext64) {
 	// Set the context pointer to the saved floating point state. This is
 	// where the guest data has been serialized, the kernel will restore
 	// from this new pointer value.
-	context.Fpstate = uint64(uintptrValue(c.floatingPointState.BytePointer()))
+	context.Fpstate = uint64(uintptrValue(c.FloatingPointState().BytePointer())) // escapes: no.
 }
diff --git a/pkg/sentry/platform/kvm/bluepill_arm64.go b/pkg/sentry/platform/kvm/bluepill_arm64.go
index acb0cb05f..df772d620 100644
--- a/pkg/sentry/platform/kvm/bluepill_arm64.go
+++ b/pkg/sentry/platform/kvm/bluepill_arm64.go
@@ -70,7 +70,7 @@ func bluepillArchExit(c *vCPU, context *arch.SignalContext64) {
 
 	lazyVfp := c.GetLazyVFP()
 	if lazyVfp != 0 {
-		fpsimd := fpsimdPtr(c.floatingPointState.BytePointer())
+		fpsimd := fpsimdPtr(c.FloatingPointState().BytePointer()) // escapes: no
 		context.Fpsimd64.Fpsr = fpsimd.Fpsr
 		context.Fpsimd64.Fpcr = fpsimd.Fpcr
 		context.Fpsimd64.Vregs = fpsimd.Vregs
@@ -90,12 +90,12 @@ func (c *vCPU) KernelSyscall() {
 
 	fpDisableTrap := ring0.CPACREL1()
 	if fpDisableTrap != 0 {
-		fpsimd := fpsimdPtr(c.floatingPointState.BytePointer())
+		fpsimd := fpsimdPtr(c.FloatingPointState().BytePointer()) // escapes: no
 		fpcr := ring0.GetFPCR()
 		fpsr := ring0.GetFPSR()
 		fpsimd.Fpcr = uint32(fpcr)
 		fpsimd.Fpsr = uint32(fpsr)
-		ring0.SaveVRegs(c.floatingPointState.BytePointer())
+		ring0.SaveVRegs(c.FloatingPointState().BytePointer()) // escapes: no
 	}
 
 	ring0.Halt()
@@ -114,12 +114,12 @@ func (c *vCPU) KernelException(vector ring0.Vector) {
 
 	fpDisableTrap := ring0.CPACREL1()
 	if fpDisableTrap != 0 {
-		fpsimd := fpsimdPtr(c.floatingPointState.BytePointer())
+		fpsimd := fpsimdPtr(c.FloatingPointState().BytePointer()) // escapes: no
 		fpcr := ring0.GetFPCR()
 		fpsr := ring0.GetFPSR()
 		fpsimd.Fpcr = uint32(fpcr)
 		fpsimd.Fpsr = uint32(fpsr)
-		ring0.SaveVRegs(c.floatingPointState.BytePointer())
+		ring0.SaveVRegs(c.FloatingPointState().BytePointer()) // escapes: no
 	}
 
 	ring0.Halt()
diff --git a/pkg/sentry/platform/kvm/machine_amd64.go b/pkg/sentry/platform/kvm/machine_amd64.go
index ab1e036b7..5bc023899 100644
--- a/pkg/sentry/platform/kvm/machine_amd64.go
+++ b/pkg/sentry/platform/kvm/machine_amd64.go
@@ -29,7 +29,6 @@ import (
 	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/ring0"
 	"gvisor.dev/gvisor/pkg/ring0/pagetables"
-	"gvisor.dev/gvisor/pkg/sentry/arch/fpu"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
 	ktime "gvisor.dev/gvisor/pkg/sentry/time"
 )
@@ -72,10 +71,6 @@ type vCPUArchState struct {
 	//
 	// This starts above fixedKernelPCID.
 	PCIDs *pagetables.PCIDs
-
-	// floatingPointState is the floating point state buffer used in guest
-	// to host transitions. See usage in bluepill_amd64.go.
-	floatingPointState fpu.State
 }
 
 const (
@@ -152,12 +147,6 @@ func (c *vCPU) initArchState() error {
 		return fmt.Errorf("error setting user registers: %v", errno)
 	}
 
-	// Allocate some floating point state save area for the local vCPU.
-	// This will be saved prior to leaving the guest, and we restore from
-	// this always. We cannot use the pointer in the context alone because
-	// we don't know how large the area there is in reality.
-	c.floatingPointState = fpu.NewState()
-
 	// Set the time offset to the host native time.
 	return c.setSystemTime()
 }
diff --git a/pkg/sentry/platform/kvm/machine_arm64.go b/pkg/sentry/platform/kvm/machine_arm64.go
index 08d98c479..31998a600 100644
--- a/pkg/sentry/platform/kvm/machine_arm64.go
+++ b/pkg/sentry/platform/kvm/machine_arm64.go
@@ -26,7 +26,6 @@ import (
 	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/ring0"
 	"gvisor.dev/gvisor/pkg/ring0/pagetables"
-	"gvisor.dev/gvisor/pkg/sentry/arch/fpu"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
 )
 
@@ -40,10 +39,6 @@ type vCPUArchState struct {
 	//
 	// This starts above fixedKernelPCID.
 	PCIDs *pagetables.PCIDs
-
-	// floatingPointState is the floating point state buffer used in guest
-	// to host transitions. See usage in bluepill_arm64.go.
-	floatingPointState fpu.State
 }
 
 const (
diff --git a/pkg/sentry/platform/kvm/machine_arm64_unsafe.go b/pkg/sentry/platform/kvm/machine_arm64_unsafe.go
index 7e8e19dcb..e73d5c544 100644
--- a/pkg/sentry/platform/kvm/machine_arm64_unsafe.go
+++ b/pkg/sentry/platform/kvm/machine_arm64_unsafe.go
@@ -28,7 +28,6 @@ import (
 	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/ring0"
 	"gvisor.dev/gvisor/pkg/ring0/pagetables"
-	"gvisor.dev/gvisor/pkg/sentry/arch/fpu"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
 	ktime "gvisor.dev/gvisor/pkg/sentry/time"
 )
@@ -159,8 +158,6 @@ func (c *vCPU) initArchState() error {
 		c.PCIDs = pagetables.NewPCIDs(fixedKernelPCID+1, poolPCIDs)
 	}
 
-	c.floatingPointState = fpu.NewState()
-
 	return c.setSystemTime()
 }