linux-kernel - Re: [PATCH v3] KVM: mmu: Fix SPTE encoding of MMIO generation upper half

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Date:   Sat, 12 Dec 2020 01:11:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>,
        stable@...gnu.org
Subject: Re: [PATCH v3] KVM: mmu: Fix SPTE encoding of MMIO generation upper
 half

On 12/12/20 00:54, Sean Christopherson wrote:
> On Fri, Dec 11, 2020, Paolo Bonzini wrote:
>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>
>> Commit cae7ed3c2cb0 ("KVM: x86: Refactor the MMIO SPTE generation handling")
>> cleaned up the computation of MMIO generation SPTE masks, however it
>> introduced a bug how the upper part was encoded:
>> SPTE bits 52-61 were supposed to contain bits 10-19 of the current
>> generation number, however a missing shift encoded bits 1-10 there instead
>> (mostly duplicating the lower part of the encoded generation number that
>> then consisted of bits 1-9).
>>
>> In the meantime, the upper part was shrunk by one bit and moved by
>> subsequent commits to become an upper half of the encoded generation number
>> (bits 9-17 of bits 0-17 encoded in a SPTE).
>>
>> In addition to the above, commit 56871d444bc4 ("KVM: x86: fix overlap between SPTE_MMIO_MASK and generation")
>> has changed the SPTE bit range assigned to encode the generation number and
>> the total number of bits encoded but did not update them in the comment
>> attached to their defines, nor in the KVM MMU doc.
>> Let's do it here, too, since it is too trivial thing to warrant a separate
>> commit.
>>
>> Fixes: cae7ed3c2cb0 ("KVM: x86: Refactor the MMIO SPTE generation handling")
>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
>> Message-Id: <156700708db2a5296c5ed7a8b9ac71f1e9765c85.1607129096.git.maciej.szmigiero@...cle.com>
>> Cc: stable@...gnu.org
> 
> I assume you want stable@...r.kernel.org?

I do.

>> [Reorganize macros so that everything is computed from the bit ranges. - Paolo]
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>> 	Compared to v2 by Maciej, I chose to keep GEN_MASK's argument calculated,
> 
> Booooo.  :-D

But I explained why. :)

Paolo

> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> 
> 
>> 	but assert on the number of bits in the low and high parts.  This is
>> 	because any change on those numbers will have to be reflected in the
>> 	comment, and essentially we're asserting that the comment is up-to-date.
>