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// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// +build amd64 386
package arch
import (
"fmt"
"gvisor.dev/gvisor/pkg/cpuid"
"gvisor.dev/gvisor/pkg/usermem"
)
// ErrFloatingPoint indicates a failed restore due to unusable floating point
// state.
type ErrFloatingPoint struct {
// supported is the supported floating point state.
supported uint64
// saved is the saved floating point state.
saved uint64
}
// Error returns a sensible description of the restore error.
func (e ErrFloatingPoint) Error() string {
return fmt.Sprintf("floating point state contains unsupported features; supported: %#x saved: %#x", e.supported, e.saved)
}
// XSTATE_BV does not exist if FXSAVE is used, but FXSAVE implicitly saves x87
// and SSE state, so this is the equivalent XSTATE_BV value.
const fxsaveBV uint64 = cpuid.XSAVEFeatureX87 | cpuid.XSAVEFeatureSSE
// afterLoadFPState is invoked by afterLoad.
func (s *State) afterLoadFPState() {
old := s.x86FPState
// Recreate the slice. This is done to ensure that it is aligned
// appropriately in memory, and large enough to accommodate any new
// state that may be saved by the new CPU. Even if extraneous new state
// is saved, the state we care about is guaranteed to be a subset of
// new state. Later optimizations can use less space when using a
// smaller state component bitmap. Intel SDM Volume 1 Chapter 13 has
// more info.
s.x86FPState = newX86FPState()
// x86FPState always contains all the FP state supported by the host.
// We may have come from a newer machine that supports additional state
// which we cannot restore.
//
// The x86 FP state areas are backwards compatible, so we can simply
// truncate the additional floating point state.
//
// Applications should not depend on the truncated state because it
// should relate only to features that were not exposed in the app
// FeatureSet. However, because we do not *prevent* them from using
// this state, we must verify here that there is no in-use state
// (according to XSTATE_BV) which we do not support.
if len(s.x86FPState) < len(old) {
// What do we support?
supportedBV := fxsaveBV
if fs := cpuid.HostFeatureSet(); fs.UseXsave() {
supportedBV = fs.ValidXCR0Mask()
}
// What was in use?
savedBV := fxsaveBV
if len(old) >= xstateBVOffset+8 {
savedBV = usermem.ByteOrder.Uint64(old[xstateBVOffset:])
}
// Supported features must be a superset of saved features.
if savedBV&^supportedBV != 0 {
panic(ErrFloatingPoint{supported: supportedBV, saved: savedBV})
}
}
// Copy to the new, aligned location.
copy(s.x86FPState, old)
}
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