[PATCH 0/2] KVM: X86: Don't reset mmu context when changing PGE or PCID

[PATCH 0/2] KVM: X86: Don't reset mmu context when changing PGE or PCID

[Lai Jiangshan]

From: Lai Jiangshan <laijs@linux.alibaba.com>

This patchset uses kvm_vcpu_flush_tlb_guest() instead of kvm_mmu_reset_context()
when X86_CR4_PGE is changed or X86_CR4_PCIDE is changed 1->0.

Neither X86_CR4_PGE nor X86_CR4_PCIDE participates in kvm_mmu_role, so
kvm_mmu_reset_context() is not required to be invoked.  Only flushing tlb
is required as SDM says.

The patchset has nothing to do with performance, because the overheads of
kvm_mmu_reset_context() and kvm_vcpu_flush_tlb_guest() are the same.  And
even in the [near] future, kvm_vcpu_flush_tlb_guest() will be optimized,
the code is not in the hot path.

This patchset makes the code more clear when to reset the mmu context.
And it makes KVM_MMU_CR4_ROLE_BITS consistent with kvm_mmu_role.

Lai Jiangshan (2):
  KVM: X86: Don't reset mmu context when X86_CR4_PCIDE 1->0
  KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE

 arch/x86/kvm/mmu.h | 5 ++---
 arch/x86/kvm/x86.c | 7 +++++--
 2 files changed, 7 insertions(+), 5 deletions(-)

-- 
2.19.1.6.gb485710b

[PATCH 1/2] KVM: X86: Don't reset mmu context when X86_CR4_PCIDE 1->0

[Lai Jiangshan]

From: Lai Jiangshan <laijs@linux.alibaba.com>

X86_CR4_PCIDE doesn't participate in kvm_mmu_role, so the mmu context
doesn't need to be reset.  It is only required to flush all the guest
tlb.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kvm/x86.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 86539c1686fa..7494ea0e7922 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -116,6 +116,7 @@ static void enter_smm(struct kvm_vcpu *vcpu);
 static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
 static void store_regs(struct kvm_vcpu *vcpu);
 static int sync_regs(struct kvm_vcpu *vcpu);
+static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu);
 
 static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
 static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
@@ -1042,9 +1043,10 @@ EXPORT_SYMBOL_GPL(kvm_is_valid_cr4);
 
 void kvm_post_set_cr4(struct kvm_vcpu *vcpu, unsigned long old_cr4, unsigned long cr4)
 {
-	if (((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS) ||
-	    (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE)))
+	if ((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS)
 		kvm_mmu_reset_context(vcpu);
+	else if (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE))
+		kvm_vcpu_flush_tlb_guest(vcpu);
 }
 EXPORT_SYMBOL_GPL(kvm_post_set_cr4);
 
-- 
2.19.1.6.gb485710b

Re: [PATCH 1/2] KVM: X86: Don't reset mmu context when X86_CR4_PCIDE 1->0

[Sean Christopherson]

On Sun, Sep 19, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> X86_CR4_PCIDE doesn't participate in kvm_mmu_role, so the mmu context
> doesn't need to be reset.  It is only required to flush all the guest
> tlb.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>  arch/x86/kvm/x86.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 86539c1686fa..7494ea0e7922 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -116,6 +116,7 @@ static void enter_smm(struct kvm_vcpu *vcpu);
>  static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
>  static void store_regs(struct kvm_vcpu *vcpu);
>  static int sync_regs(struct kvm_vcpu *vcpu);
> +static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu);
>  
>  static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>  static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> @@ -1042,9 +1043,10 @@ EXPORT_SYMBOL_GPL(kvm_is_valid_cr4);
>  
>  void kvm_post_set_cr4(struct kvm_vcpu *vcpu, unsigned long old_cr4, unsigned long cr4)
>  {
> -	if (((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS) ||
> -	    (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE)))
> +	if ((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS)
>  		kvm_mmu_reset_context(vcpu);
> +	else if (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE))
> +		kvm_vcpu_flush_tlb_guest(vcpu);

Unless there's a corner case I'm missing, I would prefer this to use
kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu) instead of calling
kvm_vcpu_flush_tlb_guest() directly.  The odds of flushes actually being batched
is low, it's more to document that kvm_post_set_cr4() _isn't_ special.

Re: [PATCH 1/2] KVM: X86: Don't reset mmu context when X86_CR4_PCIDE 1->0

[Sean Christopherson]

On Thu, Oct 14, 2021, Sean Christopherson wrote:
> On Sun, Sep 19, 2021, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@linux.alibaba.com>
> > 
> > X86_CR4_PCIDE doesn't participate in kvm_mmu_role, so the mmu context
> > doesn't need to be reset.  It is only required to flush all the guest
> > tlb.
> > 
> > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> > ---
> >  arch/x86/kvm/x86.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 86539c1686fa..7494ea0e7922 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -116,6 +116,7 @@ static void enter_smm(struct kvm_vcpu *vcpu);
> >  static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
> >  static void store_regs(struct kvm_vcpu *vcpu);
> >  static int sync_regs(struct kvm_vcpu *vcpu);
> > +static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu);
> >  
> >  static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> >  static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> > @@ -1042,9 +1043,10 @@ EXPORT_SYMBOL_GPL(kvm_is_valid_cr4);
> >  
> >  void kvm_post_set_cr4(struct kvm_vcpu *vcpu, unsigned long old_cr4, unsigned long cr4)
> >  {
> > -	if (((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS) ||
> > -	    (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE)))
> > +	if ((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS)
> >  		kvm_mmu_reset_context(vcpu);
> > +	else if (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE))
> > +		kvm_vcpu_flush_tlb_guest(vcpu);
> 
> Unless there's a corner case I'm missing, I would prefer this to use
> kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu) instead of calling
> kvm_vcpu_flush_tlb_guest() directly.  The odds of flushes actually being batched
> is low, it's more to document that kvm_post_set_cr4() _isn't_ special.

Forgot to say, with the change to KVM_REQ_TLB_FLUSH_GUEST:

Reviewed-by: Sean Christopherson <seanjc@google.com>

[PATCH 2/2] KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE

[Lai Jiangshan]

From: Lai Jiangshan <laijs@linux.alibaba.com>

X86_CR4_PGE doesn't participate in kvm_mmu_role, so the mmu context
doesn't need to be reset.  It is only required to flush all the guest
tlb.

It is also inconsistent that X86_CR4_PGE is in KVM_MMU_CR4_ROLE_BITS
while kvm_mmu_role doesn't use X86_CR4_PGE.  So X86_CR4_PGE is also
removed from KVM_MMU_CR4_ROLE_BITS.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kvm/mmu.h | 5 ++---
 arch/x86/kvm/x86.c | 3 ++-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 75367af1a6d3..e53ef2ae958f 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -44,9 +44,8 @@
 #define PT32_ROOT_LEVEL 2
 #define PT32E_ROOT_LEVEL 3
 
-#define KVM_MMU_CR4_ROLE_BITS (X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE | \
-			       X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE | \
-			       X86_CR4_LA57)
+#define KVM_MMU_CR4_ROLE_BITS (X86_CR4_PSE | X86_CR4_PAE | X86_CR4_LA57 | \
+			       X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE)
 
 #define KVM_MMU_CR0_ROLE_BITS (X86_CR0_PG | X86_CR0_WP)
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7494ea0e7922..97772e37e8ee 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1045,7 +1045,8 @@ void kvm_post_set_cr4(struct kvm_vcpu *vcpu, unsigned long old_cr4, unsigned lon
 {
 	if ((cr4 ^ old_cr4) & KVM_MMU_CR4_ROLE_BITS)
 		kvm_mmu_reset_context(vcpu);
-	else if (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE))
+	else if (((cr4 ^ old_cr4) & X86_CR4_PGE) ||
+		 (!(cr4 & X86_CR4_PCIDE) && (old_cr4 & X86_CR4_PCIDE)))
 		kvm_vcpu_flush_tlb_guest(vcpu);
 }
 EXPORT_SYMBOL_GPL(kvm_post_set_cr4);
-- 
2.19.1.6.gb485710b

Re: [PATCH 2/2] KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE

[Sean Christopherson]

On Sun, Sep 19, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> X86_CR4_PGE doesn't participate in kvm_mmu_role, so the mmu context
> doesn't need to be reset.  It is only required to flush all the guest
> tlb.
> 
> It is also inconsistent that X86_CR4_PGE is in KVM_MMU_CR4_ROLE_BITS
> while kvm_mmu_role doesn't use X86_CR4_PGE.  So X86_CR4_PGE is also
> removed from KVM_MMU_CR4_ROLE_BITS.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---

With the KVM_REQ_TLB_FLUSH_GUEST caveat, 

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH 0/2] KVM: X86: Don't reset mmu context when changing PGE or PCID

[Lai Jiangshan]

Ping

On 2021/9/19 10:42, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> This patchset uses kvm_vcpu_flush_tlb_guest() instead of kvm_mmu_reset_context()
> when X86_CR4_PGE is changed or X86_CR4_PCIDE is changed 1->0.
> 
> Neither X86_CR4_PGE nor X86_CR4_PCIDE participates in kvm_mmu_role, so
> kvm_mmu_reset_context() is not required to be invoked.  Only flushing tlb
> is required as SDM says.
> 
> The patchset has nothing to do with performance, because the overheads of
> kvm_mmu_reset_context() and kvm_vcpu_flush_tlb_guest() are the same.  And
> even in the [near] future, kvm_vcpu_flush_tlb_guest() will be optimized,
> the code is not in the hot path.
> 
> This patchset makes the code more clear when to reset the mmu context.
> And it makes KVM_MMU_CR4_ROLE_BITS consistent with kvm_mmu_role.
> 
> Lai Jiangshan (2):
>    KVM: X86: Don't reset mmu context when X86_CR4_PCIDE 1->0
>    KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE
> 
>   arch/x86/kvm/mmu.h | 5 ++---
>   arch/x86/kvm/x86.c | 7 +++++--
>   2 files changed, 7 insertions(+), 5 deletions(-)
> 

Re: [PATCH 0/2] KVM: X86: Don't reset mmu context when changing PGE or PCID

[Paolo Bonzini]

On 14/10/21 18:03, Lai Jiangshan wrote:
> Ping
> 
> On 2021/9/19 10:42, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@linux.alibaba.com>
>>
>> This patchset uses kvm_vcpu_flush_tlb_guest() instead of 
>> kvm_mmu_reset_context()
>> when X86_CR4_PGE is changed or X86_CR4_PCIDE is changed 1->0.
>>
>> Neither X86_CR4_PGE nor X86_CR4_PCIDE participates in kvm_mmu_role, so
>> kvm_mmu_reset_context() is not required to be invoked.  Only flushing tlb
>> is required as SDM says.
>>
>> The patchset has nothing to do with performance, because the overheads of
>> kvm_mmu_reset_context() and kvm_vcpu_flush_tlb_guest() are the same.  And
>> even in the [near] future, kvm_vcpu_flush_tlb_guest() will be optimized,
>> the code is not in the hot path.
>>
>> This patchset makes the code more clear when to reset the mmu context.
>> And it makes KVM_MMU_CR4_ROLE_BITS consistent with kvm_mmu_role.
>>
>> Lai Jiangshan (2):
>>    KVM: X86: Don't reset mmu context when X86_CR4_PCIDE 1->0
>>    KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE
>>
>>   arch/x86/kvm/mmu.h | 5 ++---
>>   arch/x86/kvm/x86.c | 7 +++++--
>>   2 files changed, 7 insertions(+), 5 deletions(-)
>>
> 

Queued with kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu) replacement, 
thanks.

Paolo