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

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Date:   Wed, 11 Dec 2019 23:16:41 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
CC:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: x86: use CPUID to locate host page table reserved
 bits

On Wed, 2019-12-11 at 14:48 -0600, Tom Lendacky wrote:
> On 12/11/19 3:07 AM, Paolo Bonzini wrote:
> > On 11/12/19 01:11, Huang, Kai wrote:
> > > > 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?
> > 
> > That's a question for AMD. :)  The C bit can move (and probably will,
> > otherwise they wouldn't have bothered adding it to CPUID) in future
> > generations of the processor.
> 
> Right, there's no way to guarantee that it is always below bits reported
> by CPUID. As Paolo stated, the position is reported by CPUID so that it
> can easily move and be accounted for programmatically.

Then I don't think this patch could fix the issue Paolo discribed?

Thanks,
-Kai