linux-kernel - Re: [PATCH] KVM: x86/mmu: Apply max PA check for MMIO sptes to 32-bit KVM

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Message-ID: <d2f8c8d5-39d2-6982-a4ae-eeaf4bf42658@redhat.com>
Date:   Wed, 15 Jan 2020 19:20:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Apply max PA check for MMIO sptes to 32-bit
 KVM

On 08/01/20 01:12, Sean Christopherson wrote:
> Remove the bogus 64-bit only condition from the check that disables MMIO
> spte optimization when the system supports the max PA, i.e. doesn't have
> any reserved PA bits.  32-bit KVM always uses PAE paging for the shadow
> MMU, and per Intel's SDM:
> 
>   PAE paging translates 32-bit linear addresses to 52-bit physical
>   addresses.
> 
> The kernel's restrictions on max physical addresses are limits on how
> much memory the kernel can reasonably use, not what physical addresses
> are supported by hardware.
> 
> Fixes: ce88decffd17 ("KVM: MMU: mmio page fault support")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7269130ea5e2..d9c07343d979 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6191,7 +6191,7 @@ static void kvm_set_mmio_spte_mask(void)
>  	 * If reserved bit is not supported, clear the present bit to disable
>  	 * mmio page fault.
>  	 */
> -	if (IS_ENABLED(CONFIG_X86_64) && shadow_phys_bits == 52)
> +	if (shadow_phys_bits == 52)
>  		mask &= ~1ull;
>  
>  	kvm_mmu_set_mmio_spte_mask(mask, mask, ACC_WRITE_MASK | ACC_USER_MASK);
> 

Queued, thanks.

Paolo