Re: [PATCH v2] KVM: x86: use CPUID to locate host page table reserved bits

[Huang, Kai]

On Tue, 2019-12-10 at 10:17 +0100, Paolo Bonzini wrote:
> On 04/12/19 16:57, Tom Lendacky wrote:
> > On 12/4/19 9:40 AM, 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
> > > if
> > > available, it is simplest and works even if memory is not encrypted.
> > 
> > This isn't correct for AMD. The reduction in physical addressing is
> > correct. You can't set, e.g. bit 45, in the nested page table, because
> > that will be considered a reserved bit and generate an NPF. When memory
> > encryption is enabled today, bit 47 is the encryption indicator bit and
> > bits 46:43 must be zero or else an NPF is generated. The hardware uses
> > these bits internally based on the whether running as the hypervisor or
> > based on the ASID of the guest.
> 
> kvm_get_shadow_phys_bits() must be conservative in that:
> 
> 1) if a bit is reserved it _can_ return a value higher than its index
> 
> 2) if a bit is used by the processor (for physical address or anything
> else) it _must_ return a value higher than its index.
> 
> In the SEV case we're not obeying (2), because the function returns 43
> when the C bit is bit 47.  The patch fixes that.

Could we guarantee that C-bit is always below bits reported by CPUID?

Thanks,
-Kai
> 
> Paolo
> 
> > Thanks,
> > Tom
> > 
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c | 20 ++++++++++++--------
> > >  1 file changed, 12 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 6f92b40d798c..1e4ee4f8de5f 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -538,16 +538,20 @@ 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;
> > > +	if (likely(boot_cpu_data.extended_cpuid_level >= 0x80000008))
> > > +		return cpuid_eax(0x80000008) & 0xff;
> > >  
> > > -	return cpuid_eax(0x80000008) & 0xff;
> > > +	/*
> > > +	 * Quite weird to have VMX or SVM but not MAXPHYADDR; probably a VM with
> > > +	 * custom CPUID.  Proceed with whatever the kernel found since these
> > > features
> > > +	 * aren't virtualizable (SME/SEV also require CPUIDs higher than
> > > 0x80000008).
> > > +	 */
> > > +	return boot_cpu_data.x86_phys_bits;
> > >  }
> > >  
> > >  static void kvm_mmu_reset_all_pte_masks(void)
> > >