// 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 (
	"gvisor.dev/gvisor/pkg/cpuid"
	"gvisor.dev/gvisor/pkg/usermem"
)

// 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)
}