linux-kernel - Re: [PATCH 1/2] KVM: X86: Don't reset mmu context when X86_CR4

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Date:   Thu, 14 Oct 2021 18:47:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/2] KVM: X86: Don't reset mmu context when X86_CR4_PCIDE
 1->0

On Sun, Sep 19, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.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@...ux.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.