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KVM: Lazify fpu activation and deactivation
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Defer fpu deactivation as much as possible - if the guest fpu is loaded, keep
it loaded until the next heavyweight exit (where we are forced to unload it).
This reduces unnecessary exits.

We also defer fpu activation on clts; while clts signals the intent to use the
fpu, we can't be sure the guest will actually use it.

Signed-off-by: Avi Kivity <avi@redhat.com>
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Avi Kivity authored and Marcelo Tosatti committed Mar 1, 2010
1 parent e8467fd commit 02daab2
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Showing 5 changed files with 38 additions and 31 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 @@ -506,6 +506,7 @@ struct kvm_x86_ops {
void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
void (*set_rflags)(struct kvm_vcpu *vcpu, unsigned long rflags);
void (*fpu_deactivate)(struct kvm_vcpu *vcpu);

void (*tlb_flush)(struct kvm_vcpu *vcpu);

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35 changes: 21 additions & 14 deletions arch/x86/kvm/svm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -984,17 +984,11 @@ static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
if (npt_enabled)
goto set;

if (kvm_read_cr0_bits(vcpu, X86_CR0_TS) && !(cr0 & X86_CR0_TS)) {
svm->vmcb->control.intercept_exceptions &= ~(1 << NM_VECTOR);
vcpu->fpu_active = 1;
}

vcpu->arch.cr0 = cr0;
cr0 |= X86_CR0_PG | X86_CR0_WP;
if (!vcpu->fpu_active) {
svm->vmcb->control.intercept_exceptions |= (1 << NM_VECTOR);

if (!vcpu->fpu_active)
cr0 |= X86_CR0_TS;
}
set:
/*
* re-enable caching here because the QEMU bios
Expand Down Expand Up @@ -1250,6 +1244,8 @@ static int nm_interception(struct vcpu_svm *svm)
svm->vmcb->control.intercept_exceptions &= ~(1 << NM_VECTOR);
if (!kvm_read_cr0_bits(&svm->vcpu, X86_CR0_TS))
svm->vmcb->save.cr0 &= ~X86_CR0_TS;
else
svm->vmcb->save.cr0 |= X86_CR0_TS;
svm->vcpu.fpu_active = 1;

return 1;
Expand Down Expand Up @@ -2586,6 +2582,8 @@ static void svm_flush_tlb(struct kvm_vcpu *vcpu)

static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
{
if (npt_enabled)
vcpu->fpu_active = 1;
}

static inline void sync_cr8_to_lapic(struct kvm_vcpu *vcpu)
Expand Down Expand Up @@ -2805,12 +2803,6 @@ static void svm_set_cr3(struct kvm_vcpu *vcpu, unsigned long root)

svm->vmcb->save.cr3 = root;
force_new_asid(vcpu);

if (vcpu->fpu_active) {
svm->vmcb->control.intercept_exceptions |= (1 << NM_VECTOR);
svm->vmcb->save.cr0 |= X86_CR0_TS;
vcpu->fpu_active = 0;
}
}

static int is_disabled(void)
Expand Down Expand Up @@ -2926,6 +2918,20 @@ static bool svm_rdtscp_supported(void)
return false;
}

static void svm_fpu_deactivate(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);

if (npt_enabled) {
/* hack: npt requires active fpu at this time */
vcpu->fpu_active = 1;
return;
}

svm->vmcb->control.intercept_exceptions |= 1 << NM_VECTOR;
svm->vmcb->save.cr0 |= X86_CR0_TS;
}

static struct kvm_x86_ops svm_x86_ops = {
.cpu_has_kvm_support = has_svm,
.disabled_by_bios = is_disabled,
Expand Down Expand Up @@ -2967,6 +2973,7 @@ static struct kvm_x86_ops svm_x86_ops = {
.cache_reg = svm_cache_reg,
.get_rflags = svm_get_rflags,
.set_rflags = svm_set_rflags,
.fpu_deactivate = svm_fpu_deactivate,

.tlb_flush = svm_flush_tlb,

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25 changes: 9 additions & 16 deletions arch/x86/kvm/vmx.c
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Original file line number Diff line number Diff line change
Expand Up @@ -66,7 +66,7 @@ module_param(emulate_invalid_guest_state, bool, S_IRUGO);
#define KVM_GUEST_CR0_MASK \
(KVM_GUEST_CR0_MASK_UNRESTRICTED_GUEST | X86_CR0_PG | X86_CR0_PE)
#define KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST \
(X86_CR0_WP | X86_CR0_NE | X86_CR0_TS | X86_CR0_MP)
(X86_CR0_WP | X86_CR0_NE | X86_CR0_MP)
#define KVM_VM_CR0_ALWAYS_ON \
(KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST | X86_CR0_PG | X86_CR0_PE)
#define KVM_CR4_GUEST_OWNED_BITS \
Expand Down Expand Up @@ -579,9 +579,8 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
{
u32 eb;

eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR);
if (!vcpu->fpu_active)
eb |= 1u << NM_VECTOR;
eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR)
| (1u << NM_VECTOR);
/*
* Unconditionally intercept #DB so we can maintain dr6 without
* reading it every exit.
Expand All @@ -595,6 +594,8 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
eb = ~0;
if (enable_ept)
eb &= ~(1u << PF_VECTOR); /* bypass_guest_pf = 0 */
if (vcpu->fpu_active)
eb &= ~(1u << NM_VECTOR);
vmcs_write32(EXCEPTION_BITMAP, eb);
}

Expand Down Expand Up @@ -806,9 +807,6 @@ static void vmx_fpu_activate(struct kvm_vcpu *vcpu)

static void vmx_fpu_deactivate(struct kvm_vcpu *vcpu)
{
if (!vcpu->fpu_active)
return;
vcpu->fpu_active = 0;
vmcs_set_bits(GUEST_CR0, X86_CR0_TS);
update_exception_bitmap(vcpu);
}
Expand Down Expand Up @@ -1737,8 +1735,6 @@ static void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
else
hw_cr0 = (cr0 & ~KVM_GUEST_CR0_MASK) | KVM_VM_CR0_ALWAYS_ON;

vmx_fpu_deactivate(vcpu);

if (vmx->rmode.vm86_active && (cr0 & X86_CR0_PE))
enter_pmode(vcpu);

Expand All @@ -1757,12 +1753,12 @@ static void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
if (enable_ept)
ept_update_paging_mode_cr0(&hw_cr0, cr0, vcpu);

if (!vcpu->fpu_active)
hw_cr0 |= X86_CR0_TS;

vmcs_writel(CR0_READ_SHADOW, cr0);
vmcs_writel(GUEST_CR0, hw_cr0);
vcpu->arch.cr0 = cr0;

if (!(cr0 & X86_CR0_TS) || !(cr0 & X86_CR0_PE))
vmx_fpu_activate(vcpu);
}

static u64 construct_eptp(unsigned long root_hpa)
Expand Down Expand Up @@ -1793,8 +1789,6 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)

vmx_flush_tlb(vcpu);
vmcs_writel(GUEST_CR3, guest_cr3);
if (kvm_read_cr0_bits(vcpu, X86_CR0_PE))
vmx_fpu_deactivate(vcpu);
}

static void vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
Expand Down Expand Up @@ -3002,11 +2996,9 @@ static int handle_cr(struct kvm_vcpu *vcpu)
};
break;
case 2: /* clts */
vmx_fpu_deactivate(vcpu);
vcpu->arch.cr0 &= ~X86_CR0_TS;
vmcs_writel(CR0_READ_SHADOW, kvm_read_cr0(vcpu));
trace_kvm_cr_write(0, kvm_read_cr0(vcpu));
vmx_fpu_activate(vcpu);
skip_emulated_instruction(vcpu);
return 1;
case 1: /*mov from cr*/
Expand Down Expand Up @@ -4127,6 +4119,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
.cache_reg = vmx_cache_reg,
.get_rflags = vmx_get_rflags,
.set_rflags = vmx_set_rflags,
.fpu_deactivate = vmx_fpu_deactivate,

.tlb_flush = vmx_flush_tlb,

Expand Down
7 changes: 6 additions & 1 deletion arch/x86/kvm/x86.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1509,8 +1509,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
{
kvm_x86_ops->vcpu_put(vcpu);
kvm_put_guest_fpu(vcpu);
kvm_x86_ops->vcpu_put(vcpu);
}

static int is_efer_nx(void)
Expand Down Expand Up @@ -4006,6 +4006,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
r = 0;
goto out;
}
if (test_and_clear_bit(KVM_REQ_DEACTIVATE_FPU, &vcpu->requests)) {
vcpu->fpu_active = 0;
kvm_x86_ops->fpu_deactivate(vcpu);
}
}

preempt_disable();
Expand Down Expand Up @@ -5075,6 +5079,7 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
kvm_fx_save(&vcpu->arch.guest_fx_image);
kvm_fx_restore(&vcpu->arch.host_fx_image);
++vcpu->stat.fpu_reload;
set_bit(KVM_REQ_DEACTIVATE_FPU, &vcpu->requests);
}
EXPORT_SYMBOL_GPL(kvm_put_guest_fpu);

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1 change: 1 addition & 0 deletions include/linux/kvm_host.h
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@
#define KVM_REQ_MMU_SYNC 7
#define KVM_REQ_KVMCLOCK_UPDATE 8
#define KVM_REQ_KICK 9
#define KVM_REQ_DEACTIVATE_FPU 10

#define KVM_USERSPACE_IRQ_SOURCE_ID 0

Expand Down

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