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

[Paolo Bonzini]

On 12/12/20 00:54, Sean Christopherson wrote:
> On Fri, Dec 11, 2020, Paolo Bonzini wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Message-Id: <156700708db2a5296c5ed7a8b9ac71f1e9765c85.1607129096.git.maciej.szmigiero@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxx
>
> I assume you want stable@xxxxxxxxxxxxxxx?

I do.

> > [Reorganize macros so that everything is computed from the bit ranges. - Paolo]
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> > 	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@xxxxxxxxxx>
>
>
> > 	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.