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KVM: VMX: Cache vmcs segment fields
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Since the emulator now checks segment limits and access rights, it
generates a lot more accesses to the vmcs segment fields.  Undo some
of the performance hit by cacheing those fields in a read-only cache
(the entire cache is invalidated on any write, or on guest exit).

Signed-off-by: Avi Kivity <avi@redhat.com>
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Avi Kivity committed May 22, 2011
1 parent 1aa3661 commit 2fb92db
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Showing 2 changed files with 93 additions and 10 deletions.
1 change: 1 addition & 0 deletions arch/x86/include/asm/kvm_host.h
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Original file line number Diff line number Diff line change
Expand Up @@ -136,6 +136,7 @@ enum kvm_reg_ex {
VCPU_EXREG_CR3,
VCPU_EXREG_RFLAGS,
VCPU_EXREG_CPL,
VCPU_EXREG_SEGMENTS,
};

enum {
Expand Down
102 changes: 92 additions & 10 deletions arch/x86/kvm/vmx.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -162,6 +162,10 @@ struct vcpu_vmx {
u32 ar;
} tr, es, ds, fs, gs;
} rmode;
struct {
u32 bitmask; /* 4 bits per segment (1 bit per field) */
struct kvm_save_segment seg[8];
} segment_cache;
int vpid;
bool emulation_required;

Expand All @@ -174,6 +178,15 @@ struct vcpu_vmx {
bool rdtscp_enabled;
};

enum segment_cache_field {
SEG_FIELD_SEL = 0,
SEG_FIELD_BASE = 1,
SEG_FIELD_LIMIT = 2,
SEG_FIELD_AR = 3,

SEG_FIELD_NR = 4
};

static inline struct vcpu_vmx *to_vmx(struct kvm_vcpu *vcpu)
{
return container_of(vcpu, struct vcpu_vmx, vcpu);
Expand Down Expand Up @@ -646,6 +659,62 @@ static void vmcs_set_bits(unsigned long field, u32 mask)
vmcs_writel(field, vmcs_readl(field) | mask);
}

static void vmx_segment_cache_clear(struct vcpu_vmx *vmx)
{
vmx->segment_cache.bitmask = 0;
}

static bool vmx_segment_cache_test_set(struct vcpu_vmx *vmx, unsigned seg,
unsigned field)
{
bool ret;
u32 mask = 1 << (seg * SEG_FIELD_NR + field);

if (!(vmx->vcpu.arch.regs_avail & (1 << VCPU_EXREG_SEGMENTS))) {
vmx->vcpu.arch.regs_avail |= (1 << VCPU_EXREG_SEGMENTS);
vmx->segment_cache.bitmask = 0;
}
ret = vmx->segment_cache.bitmask & mask;
vmx->segment_cache.bitmask |= mask;
return ret;
}

static u16 vmx_read_guest_seg_selector(struct vcpu_vmx *vmx, unsigned seg)
{
u16 *p = &vmx->segment_cache.seg[seg].selector;

if (!vmx_segment_cache_test_set(vmx, seg, SEG_FIELD_SEL))
*p = vmcs_read16(kvm_vmx_segment_fields[seg].selector);
return *p;
}

static ulong vmx_read_guest_seg_base(struct vcpu_vmx *vmx, unsigned seg)
{
ulong *p = &vmx->segment_cache.seg[seg].base;

if (!vmx_segment_cache_test_set(vmx, seg, SEG_FIELD_BASE))
*p = vmcs_readl(kvm_vmx_segment_fields[seg].base);
return *p;
}

static u32 vmx_read_guest_seg_limit(struct vcpu_vmx *vmx, unsigned seg)
{
u32 *p = &vmx->segment_cache.seg[seg].limit;

if (!vmx_segment_cache_test_set(vmx, seg, SEG_FIELD_LIMIT))
*p = vmcs_read32(kvm_vmx_segment_fields[seg].limit);
return *p;
}

static u32 vmx_read_guest_seg_ar(struct vcpu_vmx *vmx, unsigned seg)
{
u32 *p = &vmx->segment_cache.seg[seg].ar;

if (!vmx_segment_cache_test_set(vmx, seg, SEG_FIELD_AR))
*p = vmcs_read32(kvm_vmx_segment_fields[seg].ar_bytes);
return *p;
}

static void update_exception_bitmap(struct kvm_vcpu *vcpu)
{
u32 eb;
Expand Down Expand Up @@ -1271,9 +1340,11 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
break;
#ifdef CONFIG_X86_64
case MSR_FS_BASE:
vmx_segment_cache_clear(vmx);
vmcs_writel(GUEST_FS_BASE, data);
break;
case MSR_GS_BASE:
vmx_segment_cache_clear(vmx);
vmcs_writel(GUEST_GS_BASE, data);
break;
case MSR_KERNEL_GS_BASE:
Expand Down Expand Up @@ -1717,6 +1788,8 @@ static void enter_pmode(struct kvm_vcpu *vcpu)
vmx->emulation_required = 1;
vmx->rmode.vm86_active = 0;

vmx_segment_cache_clear(vmx);

vmcs_write16(GUEST_TR_SELECTOR, vmx->rmode.tr.selector);
vmcs_writel(GUEST_TR_BASE, vmx->rmode.tr.base);
vmcs_write32(GUEST_TR_LIMIT, vmx->rmode.tr.limit);
Expand All @@ -1740,6 +1813,8 @@ static void enter_pmode(struct kvm_vcpu *vcpu)
fix_pmode_dataseg(VCPU_SREG_GS, &vmx->rmode.gs);
fix_pmode_dataseg(VCPU_SREG_FS, &vmx->rmode.fs);

vmx_segment_cache_clear(vmx);

vmcs_write16(GUEST_SS_SELECTOR, 0);
vmcs_write32(GUEST_SS_AR_BYTES, 0x93);

Expand Down Expand Up @@ -1803,6 +1878,8 @@ static void enter_rmode(struct kvm_vcpu *vcpu)
vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
}

vmx_segment_cache_clear(vmx);

vmx->rmode.tr.selector = vmcs_read16(GUEST_TR_SELECTOR);
vmx->rmode.tr.base = vmcs_readl(GUEST_TR_BASE);
vmcs_writel(GUEST_TR_BASE, rmode_tss_base(vcpu->kvm));
Expand Down Expand Up @@ -1879,6 +1956,8 @@ static void enter_lmode(struct kvm_vcpu *vcpu)
{
u32 guest_tr_ar;

vmx_segment_cache_clear(to_vmx(vcpu));

guest_tr_ar = vmcs_read32(GUEST_TR_AR_BYTES);
if ((guest_tr_ar & AR_TYPE_MASK) != AR_TYPE_BUSY_64_TSS) {
printk(KERN_DEBUG "%s: tss fixup for long mode. \n",
Expand Down Expand Up @@ -2082,7 +2161,6 @@ static void vmx_get_segment(struct kvm_vcpu *vcpu,
struct kvm_segment *var, int seg)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
struct kvm_vmx_segment_field *sf = &kvm_vmx_segment_fields[seg];
struct kvm_save_segment *save;
u32 ar;

Expand All @@ -2104,13 +2182,13 @@ static void vmx_get_segment(struct kvm_vcpu *vcpu,
var->limit = save->limit;
ar = save->ar;
if (seg == VCPU_SREG_TR
|| var->selector == vmcs_read16(sf->selector))
|| var->selector == vmx_read_guest_seg_selector(vmx, seg))
goto use_saved_rmode_seg;
}
var->base = vmcs_readl(sf->base);
var->limit = vmcs_read32(sf->limit);
var->selector = vmcs_read16(sf->selector);
ar = vmcs_read32(sf->ar_bytes);
var->base = vmx_read_guest_seg_base(vmx, seg);
var->limit = vmx_read_guest_seg_limit(vmx, seg);
var->selector = vmx_read_guest_seg_selector(vmx, seg);
ar = vmx_read_guest_seg_ar(vmx, seg);
use_saved_rmode_seg:
if ((ar & AR_UNUSABLE_MASK) && !emulate_invalid_guest_state)
ar = 0;
Expand All @@ -2127,14 +2205,13 @@ static void vmx_get_segment(struct kvm_vcpu *vcpu,

static u64 vmx_get_segment_base(struct kvm_vcpu *vcpu, int seg)
{
struct kvm_vmx_segment_field *sf = &kvm_vmx_segment_fields[seg];
struct kvm_segment s;

if (to_vmx(vcpu)->rmode.vm86_active) {
vmx_get_segment(vcpu, &s, seg);
return s.base;
}
return vmcs_readl(sf->base);
return vmx_read_guest_seg_base(to_vmx(vcpu), seg);
}

static int __vmx_get_cpl(struct kvm_vcpu *vcpu)
Expand All @@ -2146,7 +2223,7 @@ static int __vmx_get_cpl(struct kvm_vcpu *vcpu)
&& (kvm_get_rflags(vcpu) & X86_EFLAGS_VM)) /* if virtual 8086 */
return 3;

return vmcs_read16(GUEST_CS_SELECTOR) & 3;
return vmx_read_guest_seg_selector(to_vmx(vcpu), VCPU_SREG_CS) & 3;
}

static int vmx_get_cpl(struct kvm_vcpu *vcpu)
Expand Down Expand Up @@ -2188,6 +2265,8 @@ static void vmx_set_segment(struct kvm_vcpu *vcpu,
struct kvm_vmx_segment_field *sf = &kvm_vmx_segment_fields[seg];
u32 ar;

vmx_segment_cache_clear(vmx);

if (vmx->rmode.vm86_active && seg == VCPU_SREG_TR) {
vmcs_write16(sf->selector, var->selector);
vmx->rmode.tr.selector = var->selector;
Expand Down Expand Up @@ -2229,7 +2308,7 @@ static void vmx_set_segment(struct kvm_vcpu *vcpu,

static void vmx_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l)
{
u32 ar = vmcs_read32(GUEST_CS_AR_BYTES);
u32 ar = vmx_read_guest_seg_ar(to_vmx(vcpu), VCPU_SREG_CS);

*db = (ar >> 14) & 1;
*l = (ar >> 13) & 1;
Expand Down Expand Up @@ -2816,6 +2895,8 @@ static int vmx_vcpu_reset(struct kvm_vcpu *vcpu)
if (ret != 0)
goto out;

vmx_segment_cache_clear(vmx);

seg_setup(VCPU_SREG_CS);
/*
* GUEST_CS_BASE should really be 0xffff0000, but VT vm86 mode
Expand Down Expand Up @@ -4188,6 +4269,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
| (1 << VCPU_EXREG_RFLAGS)
| (1 << VCPU_EXREG_CPL)
| (1 << VCPU_EXREG_PDPTR)
| (1 << VCPU_EXREG_SEGMENTS)
| (1 << VCPU_EXREG_CR3));
vcpu->arch.regs_dirty = 0;

Expand Down

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