linux-kernel - Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently

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Date:   Wed, 22 Nov 2023 15:42:07 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Xu Yilun <yilun.xu@...ux.intel.com>
Cc:     Paul Durrant <paul@....org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and
 use it consistently

On Wed, 2023-11-22 at 22:27 +0800, Xu Yilun wrote:
> On Wed, Nov 22, 2023 at 09:12:18AM +0000, David Woodhouse wrote:
> > On Wed, 2023-11-22 at 16:54 +0800, Xu Yilun wrote:
> > > 
> > > > @@ -259,13 +258,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
> > > >                         ret = -EFAULT;
> > > >                         goto out;
> > > >                 }
> > > > +
> > > > +               hva_change = true;
> > > > +       } else {
> > > > +               /*
> > > > +                * No need to do any re-mapping if the only thing that has
> > > > +                * changed is the page offset. Just page align it to allow the
> > > > +                * new offset to be added in.
> > > 
> > > I don't understand how the uhva('s offset) could be changed when both gpa and
> > > slot are not changed. Maybe I have no knowledge of xen, but in later
> > > patch you said your uhva would never change...
> > 
> > It doesn't change on a normal refresh with kvm_gpc_refresh(), which is
> > just for revalidation after memslot changes or MMU invalidation.
> > 
> > But it can change if the gpc is being reinitialized with a new address
> > (perhaps because the guest has made another hypercall to set the
> > address, etc.)
> > 
> > That new address could happen to be in the *same* page as the previous
> 
> In this case, the lower bits of new gpa should be different to gpc->gpa,
> so will hit "if (gpc->gpa != gpa ...)" branch.

I think that 'if (gpc->gpa != gpa); branch is also gratuitously
refreshing when it doesn't need to; it only needs to refresh if the
*aligned* gpas don't match.

But it was like that already, so I won't heckle Paul any further :)

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