Re: [PATCH v2] KVM: SVM: Override default MMIO mask if memory encryption is enabled

[Sean Christopherson]

On Fri, Dec 27, 2019 at 09:58:00AM -0600, Tom Lendacky wrote:
> The KVM MMIO support uses bit 51 as the reserved bit to cause nested page
> faults when a guest performs MMIO. The AMD memory encryption support uses
> a CPUID function to define the encryption bit position. Given this, it is
> possible that these bits can conflict.
> 
> Use svm_hardware_setup() to override the MMIO mask if memory encryption
> support is enabled. When memory encryption support is enabled the physical
> address width is reduced and the first bit after the last valid reduced
> physical address bit will always be reserved. Use this bit as the MMIO
> mask.
> 
> Fixes: 28a1f3ac1d0c ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs")
> Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  arch/x86/kvm/svm.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 122d4ce3b1ab..2cb834b5982a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1361,6 +1361,32 @@ static __init int svm_hardware_setup(void)
>  		}
>  	}
>  
> +	/*
> +	 * The default MMIO mask is a single bit (excluding the present bit),
> +	 * which could conflict with the memory encryption bit. Check for
> +	 * memory encryption support and override the default MMIO masks if
> +	 * it is enabled.
> +	 */
> +	if (cpuid_eax(0x80000000) >= 0x8000001f) {
> +		u64 msr, mask;
> +
> +		rdmsrl(MSR_K8_SYSCFG, msr);
> +		if (msr & MSR_K8_SYSCFG_MEM_ENCRYPT)  {
> +			/*
> +			 * The physical addressing width is reduced. The first
> +			 * bit above the new physical addressing limit will
> +			 * always be reserved. Use this bit and the present bit
> +			 * to generate a page fault with PFER.RSV = 1.
> +			 */
> +			mask = BIT_ULL(boot_cpu_data.x86_phys_bits);

This doesn't handle the case where x86_phys_bits _isn't_ reduced by SME/SEV
on a future processor, i.e. x86_phys_bits==52.

After staring at things for a while, I think we can handle this issue with
minimal fuss by special casing MKTME in kvm_set_mmio_spte_mask().  I'll
send a patch, I have a related bug fix for kvm_set_mmio_spte_mask() that
touches the same code.

> +			mask |= BIT_ULL(0);
> +
> +			kvm_mmu_set_mmio_spte_mask(mask, mask,
> +						   PT_WRITABLE_MASK |
> +						   PT_USER_MASK);
> +		}
> +	}
> +
>  	for_each_possible_cpu(cpu) {
>  		r = svm_cpu_init(cpu);
>  		if (r)
> -- 
> 2.17.1
>