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

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Message-ID: <b6b864e500cbb38f76739fcfb4dcc6e9c6705d0b.camel@infradead.org>
Date:   Wed, 22 Nov 2023 09:12:18 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Xu Yilun <yilun.xu@...ux.intel.com>, Paul Durrant <paul@....org>
Cc:     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 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
one. In fact the xen_shinfo_test explicitly tests that case, IIRC.

And kvm_gpc_activate() also happens to use __kvm_gpc_refresh()
internally.

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