Re: [PATCH] KVM: x86: use CPUID to locate host page table reserved bits

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Dec 04, 2019 at 03:51:00PM +0100, Paolo Bonzini wrote:
> The comment in kvm_get_shadow_phys_bits refers to MKTME, but the same is actually
> true of SME and SEV.  Just use CPUID[0x8000_0008].EAX[7:0] unconditionally, it is
> simplest and works even if memory is not encrypted.
> 
> Cc: stable@vger.kernel.org
> Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6f92b40d798c..8b8edfbdbaef 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -538,15 +538,11 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
>  static u8 kvm_get_shadow_phys_bits(void)
>  {
>  	/*
> -	 * boot_cpu_data.x86_phys_bits is reduced when MKTME is detected
> -	 * in CPU detection code, but MKTME treats those reduced bits as
> -	 * 'keyID' thus they are not reserved bits. Therefore for MKTME
> -	 * we should still return physical address bits reported by CPUID.
> +	 * boot_cpu_data.x86_phys_bits is reduced when MKTME or SME are detected
> +	 * in CPU detection code, but the processor treats those reduced bits as
> +	 * 'keyID' thus they are not reserved bits. Therefore KVM needs to look at
> +	 * the physical address bits reported by CPUID.
>  	 */
> -	if (!boot_cpu_has(X86_FEATURE_TME) ||
> -	    WARN_ON_ONCE(boot_cpu_data.extended_cpuid_level < 0x80000008))
> -		return boot_cpu_data.x86_phys_bits;

Removing this entirely will break CPUs that don't support leaf 0x80000008.
From a VMX perspective, I'm pretty sure all Intel hardware that supports
VMX is guaranteed to support 0x80000008, but I've no idea about SVM or any
non-Intel CPU, and not supporting 0x80000008 in a virtual machine is
technically legal/possible.  We conditioned doing CPUID on TME because TME
would be reported as supported iff 0x80000008 existed.

The extra bit of paranoia doesn't cost much, so play it safe?  E.g.:

	if (unlikely(boot_cpu_data.extended_cpuid_level < 0x80000008)) {
		WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_TME) || SME?);
		return boot_cpu_data.x86_phys_bits;
	}

	return cpuid_eax(0x80000008) & 0xff;

> -
>  	return cpuid_eax(0x80000008) & 0xff;
>  }
>  
> -- 
> 1.8.3.1
>