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Make SyscallTable.lookup be a fixed-size array rather than a slice.

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Same for `SyscallFlagsTable.enable`.

This avoids a few extra memory reads during syscall execution, at the expense
of holding two slightly-larger-than-necessary-in-practice lookup arrays in
memory. Syscall latency overhead should be improved very slightly.

PiperOrigin-RevId: 419898650
This commit is contained in:
Etienne Perot
2022-01-05 13:23:12 -08:00
committed by gVisor bot
parent 496ba59add
commit d057161dc8
1 changed files with 14 additions and 17 deletions
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pkg/sentry/kernel/syscalls.go
+14 -17
View File
@@ -25,12 +25,14 @@ import (
"gvisor.dev/gvisor/pkg/sync"
)
// maxSyscallNum is the highest supported syscall number.
//
// The types below create fast lookup slices for all syscalls. This maximum
// serves as a sanity check that we don't allocate huge slices for a very large
// syscall. This is checked during registration.
const maxSyscallNum = 2000
const (
// maxSyscallNum is the highest supported syscall number.
//
// The types below create fast lookup slices for all syscalls. This maximum
// serves as a sanity check that we don't allocate huge slices for a very large
// syscall. This is checked during registration.
maxSyscallNum = 2000
)
// SyscallSupportLevel is a syscall support levels.
type SyscallSupportLevel int
@@ -119,7 +121,7 @@ type SyscallFlagsTable struct {
//
// missing syscalls have the same value in enable as missingEnable to
// avoid an extra branch in Word.
enable []uint32
enable [maxSyscallNum + 1]uint32
// missingEnable contains the enable bits for missing syscalls.
missingEnable uint32
@@ -128,8 +130,7 @@ type SyscallFlagsTable struct {
// Init initializes the struct, with all syscalls in table set to enable.
//
// max is the largest syscall number in table.
func (e *SyscallFlagsTable) init(table map[uintptr]Syscall, max uintptr) {
e.enable = make([]uint32, max+1)
func (e *SyscallFlagsTable) init(table map[uintptr]Syscall) {
for num := range table {
e.enable[num] = syscallPresent
}
@@ -137,7 +138,7 @@ func (e *SyscallFlagsTable) init(table map[uintptr]Syscall, max uintptr) {
// Word returns the enable bitfield for sysno.
func (e *SyscallFlagsTable) Word(sysno uintptr) uint32 {
if sysno < uintptr(len(e.enable)) {
if sysno <= maxSyscallNum {
return atomic.LoadUint32(&e.enable[sysno])
}
@@ -239,7 +240,7 @@ type SyscallTable struct {
// lookup is a fixed-size array that holds the syscalls (indexed by
// their numbers). It is used for fast look ups.
lookup []SyscallFn
lookup [maxSyscallNum + 1]SyscallFn
// Emulate is a collection of instruction addresses to emulate. The
// keys are addresses, and the values are system call numbers.
@@ -319,24 +320,20 @@ func (s *SyscallTable) Init() {
s.Emulate = make(map[hostarch.Addr]uintptr)
}
max := s.MaxSysno() // Checked during RegisterSyscallTable.
// Initialize the fast-lookup table.
s.lookup = make([]SyscallFn, max+1)
for num, sc := range s.Table {
s.lookup[num] = sc.Fn
}
// Initialize all features.
s.FeatureEnable.init(s.Table, max)
s.FeatureEnable.init(s.Table)
}
// Lookup returns the syscall implementation, if one exists.
func (s *SyscallTable) Lookup(sysno uintptr) SyscallFn {
if sysno < uintptr(len(s.lookup)) {
if sysno <= maxSyscallNum {
return s.lookup[sysno]
}
return nil
}