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

[Sean Christopherson <seanjc@google.com>]

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>