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

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Message-ID: <f0028b5a-101a-4f19-80b8-a00926bc68dd@default>
Date:   Sun, 13 May 2018 01:03:53 -0700 (PDT)
From:   Liran Alon <liran.alon@...cle.com>
To:     <kernellwp@...il.com>
Cc:     <rkrcmar@...hat.com>, <pbonzini@...hat.com>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <junaids@...gle.com>
Subject: Re: [PATCH 2/2] KVM: X86: Fix loss of CR3_PCID_INVD bit when guest
 writes CR3


----- kernellwp@...il.com 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))
> -- 
> 2.7.4

This commit doesn't seem correct to me.

According to Intel SDM "MOV—Move to/from Control Registers":
"If CR4.PCIDE = 1, bit 63 of the source operand to MOV to CR3 determines whether the instruction
invalidates entries in the TLBs and the paging-structure caches
(see Section 4.10.4.1, “Operations that Invalidate TLBs and Paging-Structure Caches,”
in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A).
The instruction does not modify bit 63 of CR3, which is reserved and always 0."

However, after this commit kvm_set_cr3() will update vcpu->arch.cr3 to have bit CR3_PCID_INVD set.
Which is wrong as it should be reserved and always 0.