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

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Date:   Sun, 6 Dec 2020 11:09:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
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 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)


Thanks,

Paolo