Re: [PATCH] KVM: x86/mmu: Set mmio_value to '0' if reserved #PF can't be generated

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Set the mmio_value to '0' instead of simply clearing the present bit to
> squash a benign warning in kvm_mmu_set_mmio_spte_mask() that complains
> about the mmio_value overlapping the lower GFN mask on systems with 52
> bits of PA space.
>
> Opportunistically clean up the code and comments.
>
> Fixes: 608831174100 ("KVM: x86: only do L1TF workaround on affected processors")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>
> Thanks for the excuse to clean up kvm_set_mmio_spte_mask(), been wanting a
> reason to fix that mess for a few months now :-).
>
>  arch/x86/kvm/mmu/mmu.c | 27 +++++++++------------------
>  1 file changed, 9 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 2df0f347655a4..aab90f4079ea9 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6136,25 +6136,16 @@ static void kvm_set_mmio_spte_mask(void)
>  	u64 mask;
>  
>  	/*
> -	 * Set the reserved bits and the present bit of an paging-structure
> -	 * entry to generate page fault with PFER.RSV = 1.
> +	 * Set a reserved PA bit in MMIO SPTEs to generate page faults with
> +	 * PFEC.RSVD=1 on MMIO accesses.  64-bit PTEs (PAE, x86-64, and EPT
> +	 * paging) support a maximum of 52 bits of PA, i.e. if the CPU supports
> +	 * 52-bit physical addresses then there are no reserved PA bits in the
> +	 * PTEs and so the reserved PA approach must be disabled.
>  	 */
> -
> -	/*
> -	 * Mask the uppermost physical address bit, which would be reserved as
> -	 * long as the supported physical address width is less than 52.
> -	 */
> -	mask = 1ull << 51;
> -
> -	/* Set the present bit. */
> -	mask |= 1ull;
> -
> -	/*
> -	 * If reserved bit is not supported, clear the present bit to disable
> -	 * mmio page fault.
> -	 */
> -	if (shadow_phys_bits == 52)
> -		mask &= ~1ull;
> +	if (shadow_phys_bits < 52)
> +		mask = BIT_ULL(51) | PT_PRESENT_MASK;
> +	else
> +		mask = 0;
>  
>  	kvm_mmu_set_mmio_spte_mask(mask, mask, ACC_WRITE_MASK | ACC_USER_MASK);
>  }

Nice cleanup,

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly