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KVM: x86 emulator: lazily evaluate segment registers
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Instead of prefetching all segment bases before emulation, read them at the
last moment.  Since most of them are unneeded, we save some cycles on
Intel machines where this is a bit expensive.

Signed-off-by: Avi Kivity <avi@qumranet.com>
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Avi Kivity committed Jul 20, 2008
1 parent 0adc867 commit 7a5b56d
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Showing 3 changed files with 60 additions and 67 deletions.
21 changes: 0 additions & 21 deletions arch/x86/kvm/x86.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2126,27 +2126,6 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
? X86EMUL_MODE_PROT64 : cs_db
? X86EMUL_MODE_PROT32 : X86EMUL_MODE_PROT16;

if (vcpu->arch.emulate_ctxt.mode == X86EMUL_MODE_PROT64) {
vcpu->arch.emulate_ctxt.cs_base = 0;
vcpu->arch.emulate_ctxt.ds_base = 0;
vcpu->arch.emulate_ctxt.es_base = 0;
vcpu->arch.emulate_ctxt.ss_base = 0;
} else {
vcpu->arch.emulate_ctxt.cs_base =
get_segment_base(vcpu, VCPU_SREG_CS);
vcpu->arch.emulate_ctxt.ds_base =
get_segment_base(vcpu, VCPU_SREG_DS);
vcpu->arch.emulate_ctxt.es_base =
get_segment_base(vcpu, VCPU_SREG_ES);
vcpu->arch.emulate_ctxt.ss_base =
get_segment_base(vcpu, VCPU_SREG_SS);
}

vcpu->arch.emulate_ctxt.gs_base =
get_segment_base(vcpu, VCPU_SREG_GS);
vcpu->arch.emulate_ctxt.fs_base =
get_segment_base(vcpu, VCPU_SREG_FS);

r = x86_decode_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);

/* Reject the instructions other than VMCALL/VMMCALL when
Expand Down
96 changes: 57 additions & 39 deletions arch/x86/kvm/x86_emulate.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -522,6 +522,39 @@ static inline void jmp_rel(struct decode_cache *c, int rel)
register_address_increment(c, &c->eip, rel);
}

static void set_seg_override(struct decode_cache *c, int seg)
{
c->has_seg_override = true;
c->seg_override = seg;
}

static unsigned long seg_base(struct x86_emulate_ctxt *ctxt, int seg)
{
if (ctxt->mode == X86EMUL_MODE_PROT64 && seg < VCPU_SREG_FS)
return 0;

return kvm_x86_ops->get_segment_base(ctxt->vcpu, seg);
}

static unsigned long seg_override_base(struct x86_emulate_ctxt *ctxt,
struct decode_cache *c)
{
if (!c->has_seg_override)
return 0;

return seg_base(ctxt, c->seg_override);
}

static unsigned long es_base(struct x86_emulate_ctxt *ctxt)
{
return seg_base(ctxt, VCPU_SREG_ES);
}

static unsigned long ss_base(struct x86_emulate_ctxt *ctxt)
{
return seg_base(ctxt, VCPU_SREG_SS);
}

static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt,
struct x86_emulate_ops *ops,
unsigned long linear, u8 *dest)
Expand Down Expand Up @@ -735,8 +768,8 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
}
if (c->modrm_rm == 2 || c->modrm_rm == 3 ||
(c->modrm_rm == 6 && c->modrm_mod != 0))
if (!c->override_base)
c->override_base = &ctxt->ss_base;
if (!c->has_seg_override)
set_seg_override(c, VCPU_SREG_SS);
c->modrm_ea = (u16)c->modrm_ea;
} else {
/* 32/64-bit ModR/M decode. */
Expand Down Expand Up @@ -807,6 +840,7 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)

memset(c, 0, sizeof(struct decode_cache));
c->eip = ctxt->vcpu->arch.rip;
ctxt->cs_base = seg_base(ctxt, VCPU_SREG_CS);
memcpy(c->regs, ctxt->vcpu->arch.regs, sizeof c->regs);

switch (mode) {
Expand Down Expand Up @@ -845,23 +879,15 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
/* switch between 2/4 bytes */
c->ad_bytes = def_ad_bytes ^ 6;
break;
case 0x26: /* ES override */
case 0x2e: /* CS override */
c->override_base = &ctxt->cs_base;
break;
case 0x36: /* SS override */
case 0x3e: /* DS override */
c->override_base = &ctxt->ds_base;
break;
case 0x26: /* ES override */
c->override_base = &ctxt->es_base;
set_seg_override(c, (c->b >> 3) & 3);
break;
case 0x64: /* FS override */
c->override_base = &ctxt->fs_base;
break;
case 0x65: /* GS override */
c->override_base = &ctxt->gs_base;
break;
case 0x36: /* SS override */
c->override_base = &ctxt->ss_base;
set_seg_override(c, c->b & 7);
break;
case 0x40 ... 0x4f: /* REX */
if (mode != X86EMUL_MODE_PROT64)
Expand Down Expand Up @@ -933,15 +959,11 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
if (rc)
goto done;

if (!c->override_base)
c->override_base = &ctxt->ds_base;
if (mode == X86EMUL_MODE_PROT64 &&
c->override_base != &ctxt->fs_base &&
c->override_base != &ctxt->gs_base)
c->override_base = NULL;
if (!c->has_seg_override)
set_seg_override(c, VCPU_SREG_DS);

if (c->override_base && !(!c->twobyte && c->b == 0x8d))
c->modrm_ea += *c->override_base;
if (!(!c->twobyte && c->b == 0x8d))
c->modrm_ea += seg_override_base(ctxt, c);

if (c->ad_bytes != 8)
c->modrm_ea = (u32)c->modrm_ea;
Expand Down Expand Up @@ -1043,7 +1065,7 @@ static inline void emulate_push(struct x86_emulate_ctxt *ctxt)
c->dst.bytes = c->op_bytes;
c->dst.val = c->src.val;
register_address_increment(c, &c->regs[VCPU_REGS_RSP], -c->op_bytes);
c->dst.ptr = (void *) register_address(c, ctxt->ss_base,
c->dst.ptr = (void *) register_address(c, ss_base(ctxt),
c->regs[VCPU_REGS_RSP]);
}

Expand All @@ -1053,7 +1075,7 @@ static inline int emulate_grp1a(struct x86_emulate_ctxt *ctxt,
struct decode_cache *c = &ctxt->decode;
int rc;

rc = ops->read_std(register_address(c, ctxt->ss_base,
rc = ops->read_std(register_address(c, ss_base(ctxt),
c->regs[VCPU_REGS_RSP]),
&c->dst.val, c->dst.bytes, ctxt->vcpu);
if (rc != 0)
Expand Down Expand Up @@ -1375,11 +1397,11 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
register_address_increment(c, &c->regs[VCPU_REGS_RSP],
-c->op_bytes);
c->dst.ptr = (void *) register_address(
c, ctxt->ss_base, c->regs[VCPU_REGS_RSP]);
c, ss_base(ctxt), c->regs[VCPU_REGS_RSP]);
break;
case 0x58 ... 0x5f: /* pop reg */
pop_instruction:
if ((rc = ops->read_std(register_address(c, ctxt->ss_base,
if ((rc = ops->read_std(register_address(c, ss_base(ctxt),
c->regs[VCPU_REGS_RSP]), c->dst.ptr,
c->op_bytes, ctxt->vcpu)) != 0)
goto done;
Expand All @@ -1405,7 +1427,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->rep_prefix ?
address_mask(c, c->regs[VCPU_REGS_RCX]) : 1,
(ctxt->eflags & EFLG_DF),
register_address(c, ctxt->es_base,
register_address(c, es_base(ctxt),
c->regs[VCPU_REGS_RDI]),
c->rep_prefix,
c->regs[VCPU_REGS_RDX]) == 0) {
Expand All @@ -1421,9 +1443,8 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->rep_prefix ?
address_mask(c, c->regs[VCPU_REGS_RCX]) : 1,
(ctxt->eflags & EFLG_DF),
register_address(c, c->override_base ?
*c->override_base :
ctxt->ds_base,
register_address(c,
seg_override_base(ctxt, c),
c->regs[VCPU_REGS_RSI]),
c->rep_prefix,
c->regs[VCPU_REGS_RDX]) == 0) {
Expand Down Expand Up @@ -1559,11 +1580,10 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->dst.type = OP_MEM;
c->dst.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
c->dst.ptr = (unsigned long *)register_address(c,
ctxt->es_base,
es_base(ctxt),
c->regs[VCPU_REGS_RDI]);
if ((rc = ops->read_emulated(register_address(c,
c->override_base ? *c->override_base :
ctxt->ds_base,
seg_override_base(ctxt, c),
c->regs[VCPU_REGS_RSI]),
&c->dst.val,
c->dst.bytes, ctxt->vcpu)) != 0)
Expand All @@ -1579,8 +1599,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->src.type = OP_NONE; /* Disable writeback. */
c->src.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
c->src.ptr = (unsigned long *)register_address(c,
c->override_base ? *c->override_base :
ctxt->ds_base,
seg_override_base(ctxt, c),
c->regs[VCPU_REGS_RSI]);
if ((rc = ops->read_emulated((unsigned long)c->src.ptr,
&c->src.val,
Expand All @@ -1591,7 +1610,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->dst.type = OP_NONE; /* Disable writeback. */
c->dst.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
c->dst.ptr = (unsigned long *)register_address(c,
ctxt->es_base,
es_base(ctxt),
c->regs[VCPU_REGS_RDI]);
if ((rc = ops->read_emulated((unsigned long)c->dst.ptr,
&c->dst.val,
Expand All @@ -1615,7 +1634,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->dst.type = OP_MEM;
c->dst.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
c->dst.ptr = (unsigned long *)register_address(c,
ctxt->es_base,
es_base(ctxt),
c->regs[VCPU_REGS_RDI]);
c->dst.val = c->regs[VCPU_REGS_RAX];
register_address_increment(c, &c->regs[VCPU_REGS_RDI],
Expand All @@ -1627,8 +1646,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
c->dst.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
c->dst.ptr = (unsigned long *)&c->regs[VCPU_REGS_RAX];
if ((rc = ops->read_emulated(register_address(c,
c->override_base ? *c->override_base :
ctxt->ds_base,
seg_override_base(ctxt, c),
c->regs[VCPU_REGS_RSI]),
&c->dst.val,
c->dst.bytes,
Expand Down
10 changes: 3 additions & 7 deletions include/asm-x86/kvm_x86_emulate.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -124,7 +124,8 @@ struct decode_cache {
u8 rex_prefix;
struct operand src;
struct operand dst;
unsigned long *override_base;
bool has_seg_override;
u8 seg_override;
unsigned int d;
unsigned long regs[NR_VCPU_REGS];
unsigned long eip;
Expand All @@ -151,12 +152,7 @@ struct x86_emulate_ctxt {
/* Emulated execution mode, represented by an X86EMUL_MODE value. */
int mode;

unsigned long cs_base;
unsigned long ds_base;
unsigned long es_base;
unsigned long ss_base;
unsigned long gs_base;
unsigned long fs_base;
u32 cs_base;

/* decode cache */

Expand Down

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