linux-kernel - Re: [PATCH 2/2] KVM: X86: Fix loss of CR3_PCID

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Date:   Sun, 15 Jul 2018 18:05:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH 2/2] KVM: X86: Fix loss of CR3_PCID_INVD bit when guest
 writes CR3

On 13/05/2018 05:24, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> SDM volume 3, section 4.10.4:
> 
> * MOV to CR3. The behavior of the instruction depends on the value of CR4.PCIDE:
> — If CR4.PCIDE = 1 and bit 63 of the instruction’s source operand is 1, the 
>   instruction is not required to invalidate any TLB entries or entries in 
>   paging-structure caches.
> 
> The CR3_PCID_INVD bit should not be removed if CR4.PCIDE = 1 when guest writes 
> CR3, this patch fixes it.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Junaid Shahid <junaids@...gle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.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 9a90668..438f140 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -849,11 +849,13 @@ EXPORT_SYMBOL_GPL(kvm_set_cr4);
>  
>  int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  {
> +	unsigned long cr3_check = cr3;
> +
>  #ifdef CONFIG_X86_64
>  	bool pcid_enabled = kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE);
>  
>  	if (pcid_enabled)
> -		cr3 &= ~CR3_PCID_INVD;
> +		cr3_check &= ~CR3_PCID_INVD;
>  #endif
>  
>  	if (cr3 == kvm_read_cr3(vcpu) && !pdptrs_changed(vcpu)) {
> @@ -863,7 +865,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  	}
>  
>  	if (is_long_mode(vcpu) &&
> -	    (cr3 & rsvd_bits(cpuid_maxphyaddr(vcpu), 63)))
> +	    (cr3_check & rsvd_bits(cpuid_maxphyaddr(vcpu), 63)))
>  		return 1;
>  	else if (is_pae(vcpu) && is_paging(vcpu) &&
>  		   !load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
> 

Note that bit 63 is never written into CR3, and is always zero when read.

Paolo