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

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Date:   Sun, 6 Dec 2020 16:53:42 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: mmu: Fix SPTE encoding of MMIO generation upper half

On 06.12.2020 11:09, Paolo Bonzini wrote:
> On 05/12/20 01:48, Maciej S. Szmigiero 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>
>> ---
> 
> 
> Good catch.  What do you think about this alternative definition?  It computes everything from the bit ranges.
> 
> #define MMIO_SPTE_GEN_LOW_START         3
> #define MMIO_SPTE_GEN_LOW_END           11
> 
> #define MMIO_SPTE_GEN_HIGH_START        PT64_SECOND_AVAIL_BITS_SHIFT
> #define MMIO_SPTE_GEN_HIGH_END          62
> 
> #define MMIO_SPTE_GEN_LOW_MASK          GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
> 
> MMIO_SPTE_GEN_LOW_START)
> #define MMIO_SPTE_GEN_HIGH_MASK GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
> 
> MMIO_SPTE_GEN_HIGH_START)
> 
> #define MMIO_SPTE_GEN_LOW_BITS          (MMIO_SPTE_GEN_LOW_END - MMIO_SPTE_GEN_LOW_START + 1)
> #define MMIO_SPTE_GEN_HIGH_BITS         (MMIO_SPTE_GEN_HIGH_END - MMIO_SPTE_GEN_HIGH_START + 1)
> 
> #define MMIO_SPTE_GEN_LOW_SHIFT         (MMIO_SPTE_GEN_LOW_START - 0)
> #define MMIO_SPTE_GEN_HIGH_SHIFT        (MMIO_SPTE_GEN_HIGH_START - MMIO_SPTE_GEN_LOW_BITS)
> 
> #define MMIO_SPTE_GEN_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_BITS + MMIO_SPTE_GEN_HIGH_BITS - 1, 0)

I like the exiting version more since it explicitly refers to start bits
0 and 9 of the encoded generation for easy cross-checking with bit ranges
in the comment above these defines in spte.h.

But if you prefer it to be specified as you had proposed above I will respin
the patch accordingly.

> Thanks,
> 
> Paolo
> 

Thanks,
Maciej