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// Copyright 2018 Google Inc.
//
// 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 i386 amd64
package pagetables
import (
"sync"
)
// maxPCID is the maximum allowed PCID.
const maxPCID = 4095
// PCIDs is a simple PCID database.
type PCIDs struct {
mu sync.Mutex
// last is the last fresh PCID given out (not including the available
// pool). If last >= maxPCID, then the only PCIDs available in the
// available pool below.
last uint16
// available are PCIDs that have been freed.
available map[uint16]struct{}
}
// NewPCIDs returns a new PCID set.
func NewPCIDs() *PCIDs {
return &PCIDs{
available: make(map[uint16]struct{}),
}
}
// allocate returns an unused PCID, or zero if all are taken.
func (p *PCIDs) allocate() uint16 {
p.mu.Lock()
defer p.mu.Unlock()
if len(p.available) > 0 {
for id := range p.available {
delete(p.available, id)
return id
}
}
if id := p.last + 1; id <= maxPCID {
p.last = id
return id
}
// Nothing available.
return 0
}
// free returns a PCID to the pool.
//
// It is safe to call free with a zero pcid. That is, you may always call free
// with anything returned by allocate.
func (p *PCIDs) free(id uint16) {
p.mu.Lock()
defer p.mu.Unlock()
if id != 0 {
p.available[id] = struct{}{}
}
}
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