diff mbox series

KVM: x86: Optimization: Requst TLB flush in fast_cr3_switch() instead of do it directly

Message ID 20191112183300.6959-1-liran.alon@oracle.com (mailing list archive)
State New, archived
Headers show
Series KVM: x86: Optimization: Requst TLB flush in fast_cr3_switch() instead of do it directly | expand

Commit Message

Liran Alon Nov. 12, 2019, 6:33 p.m. UTC 
When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
page. If nEPT is used, this will happen from
kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
__kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
previously cached root page.

In case fast_cr3_switch() finds a matching cached root page, it will
set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
next entry to guest, KVM will set root HPA in appropriate hardware
fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
was replaced.

This works as mmu->root_hpa, which vmx_flush_tlb() use, was
already replaced in cached_root_available(). However, this may
result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
may have already been requested. For example, by prepare_vmcs02()
in case L1 don't use VPID.

Therefore, change fast_cr3_switch() to just request TLB flush on
next entry to guest.

Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
---
 arch/x86/kvm/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Sean Christopherson Nov. 12, 2019, 7:02 p.m. UTC | #1 
On Tue, Nov 12, 2019 at 08:33:00PM +0200, Liran Alon wrote:
> When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
> page. If nEPT is used, this will happen from
> kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
> happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
> __kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
> previously cached root page.
> 
> In case fast_cr3_switch() finds a matching cached root page, it will
> set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
> next entry to guest, KVM will set root HPA in appropriate hardware
> fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
> kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
> was replaced.
> 
> This works as mmu->root_hpa, which vmx_flush_tlb() use, was
> already replaced in cached_root_available(). However, this may
> result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
> may have already been requested. For example, by prepare_vmcs02()
> in case L1 don't use VPID.
> 
> Therefore, change fast_cr3_switch() to just request TLB flush on
> next entry to guest.
> 
> Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 24c23c66b226..150d982ec1d2 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4295,7 +4295,7 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
>  			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
>  			if (!skip_tlb_flush) {
>  				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> -				kvm_x86_ops->tlb_flush(vcpu, true);
> +				kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);

Ha, I brought this up in the original review[*].  Junaid was concerned with
maintaining the existing behavior for vcpu->stat.tlb_flush, so we kept the
direct call to ->tlb_flush().  Incrementing tlb_flush seems a-ok, so:

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

[*] https://patchwork.kernel.org/patch/10461319/#21985483

>  			}
>  
>  			/*
> -- 
> 2.20.1
>
Vitaly Kuznetsov Nov. 13, 2019, 9:20 a.m. UTC | #2 
Liran Alon <liran.alon@oracle.com> writes:

> When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
> page. If nEPT is used, this will happen from
> kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
> happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
> __kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
> previously cached root page.
>
> In case fast_cr3_switch() finds a matching cached root page, it will
> set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
> next entry to guest, KVM will set root HPA in appropriate hardware
> fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
> kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
> was replaced.
>
> This works as mmu->root_hpa, which vmx_flush_tlb() use, was
> already replaced in cached_root_available(). However, this may
> result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
> may have already been requested. For example, by prepare_vmcs02()
> in case L1 don't use VPID.
>
> Therefore, change fast_cr3_switch() to just request TLB flush on
> next entry to guest.
>
> Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 24c23c66b226..150d982ec1d2 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4295,7 +4295,7 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
>  			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
>  			if (!skip_tlb_flush) {
>  				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> -				kvm_x86_ops->tlb_flush(vcpu, true);
> +				kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>  			}
>  
>  			/*

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Paolo Bonzini Nov. 13, 2019, 3:17 p.m. UTC | #3 
On 12/11/19 19:33, Liran Alon wrote:
> When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
> page. If nEPT is used, this will happen from
> kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
> happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
> __kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
> previously cached root page.
> 
> In case fast_cr3_switch() finds a matching cached root page, it will
> set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
> next entry to guest, KVM will set root HPA in appropriate hardware
> fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
> kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
> was replaced.
> 
> This works as mmu->root_hpa, which vmx_flush_tlb() use, was
> already replaced in cached_root_available(). However, this may
> result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
> may have already been requested. For example, by prepare_vmcs02()
> in case L1 don't use VPID.
> 
> Therefore, change fast_cr3_switch() to just request TLB flush on
> next entry to guest.
> 
> Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 24c23c66b226..150d982ec1d2 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4295,7 +4295,7 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
>  			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
>  			if (!skip_tlb_flush) {
>  				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> -				kvm_x86_ops->tlb_flush(vcpu, true);
> +				kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>  			}
>  
>  			/*
> 

Queued, thanks.

(I should get kvm/queue properly tested and pushed by the end of this week).

Paolo
Liran Alon Nov. 13, 2019, 3:20 p.m. UTC | #4 
> On 13 Nov 2019, at 17:17, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> On 12/11/19 19:33, Liran Alon wrote:
>> When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
>> page. If nEPT is used, this will happen from
>> kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
>> happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
>> __kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
>> previously cached root page.
>> 
>> In case fast_cr3_switch() finds a matching cached root page, it will
>> set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
>> next entry to guest, KVM will set root HPA in appropriate hardware
>> fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
>> kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
>> was replaced.
>> 
>> This works as mmu->root_hpa, which vmx_flush_tlb() use, was
>> already replaced in cached_root_available(). However, this may
>> result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
>> may have already been requested. For example, by prepare_vmcs02()
>> in case L1 don't use VPID.
>> 
>> Therefore, change fast_cr3_switch() to just request TLB flush on
>> next entry to guest.
>> 
>> Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
>> Signed-off-by: Liran Alon <liran.alon@oracle.com>
>> ---
>> arch/x86/kvm/mmu.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 24c23c66b226..150d982ec1d2 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -4295,7 +4295,7 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
>> 			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
>> 			if (!skip_tlb_flush) {
>> 				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
>> -				kvm_x86_ops->tlb_flush(vcpu, true);
>> +				kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>> 			}
>> 
>> 			/*
>> 
> 
> Queued, thanks.
> 
> (I should get kvm/queue properly tested and pushed by the end of this week).
> 
> Paolo
> 

Thanks.

Also note that I have sent another trivial patch that didn’t got any response ("KVM: VMX: Consume pending LAPIC INIT event when exit on INIT_SIGNAL”).
See: https://patchwork.kernel.org/patch/11236869/

I have also sent kvm-unit-tests for my recent patches (The INIT_SIGNAL fix and nVMX TPR-Threshold issue).
See: https://patchwork.kernel.org/patch/11236951/ and https://patchwork.kernel.org/patch/11236961/

-Liran
diff mbox series

Patch

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 24c23c66b226..150d982ec1d2 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4295,7 +4295,7 @@  static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
 			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
 			if (!skip_tlb_flush) {
 				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-				kvm_x86_ops->tlb_flush(vcpu, true);
+				kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
 			}
 
 			/*