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

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Message-ID: <62438ac9-e186-32a7-d12f-5806054d56b2@redhat.com>
Date:   Wed, 11 Dec 2019 10:07:55 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
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 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.

Paolo