Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio generation number

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/08/2014 10:50, Xiao Guangrong ha scritto:
> Okay, what confused me it that it seems that the single line patch
> is ok to you. :)

No, it was late and I was confused. :)

> Now, do we really need to care the case 2? like David said:
> "Sorry I didn't explain myself very well: Since we can get a single wrong
> mmio exit no matter what, it has to be handled in userspace. So my point
> was, it doesn't really help to fix that one very specific way that it can
> happen, because it can just happen in other ways. (E.g. update memslots
> occurs after is_noslot_pfn() and before mmio exit)."
> 
> What's your idea?
> 
> > I think if you always treat the low bit as zero in mmio sptes, you can 
> > do that without losing a bit of the generation.
> 
> What's you did is avoiding cache a invalid generation number into spte, but
> actually if we can figure it out when we check mmio access, it's ok. Like the
> updated patch i posted should fix it, that way avoids doubly increase the number.

Yes.

> Okay, if you're interested increasing the number doubly, there is the simpler
> one:

This wastes a bit in the mmio spte though.  My idea is to increase the
memslots generation twice, but drop the low bit in the mmio spte.

Paolo