linux-kernel - Re: [PATCH 26/27] KVM: x86/mmu: Add page-track API to query if a gfn is valid

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Message-ID: <Y7cOt5R/wK2Y1fg5@google.com>
Date:   Thu, 5 Jan 2023 17:53:59 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 26/27] KVM: x86/mmu: Add page-track API to query if a gfn
 is valid

On Thu, Jan 05, 2023, Yan Zhao wrote:
> On Tue, Jan 03, 2023 at 09:19:01PM +0000, Sean Christopherson wrote:
> > On Wed, Dec 28, 2022, Yan Zhao wrote:
> > > On Fri, Dec 23, 2022 at 12:57:38AM +0000, Sean Christopherson wrote:
> > > > +bool kvm_page_track_is_valid_gfn(struct kvm *kvm, gfn_t gfn)
> > > > +{
> > > > +	bool ret;
> > > > +	int idx;
> > > > +
> > > > +	idx = srcu_read_lock(&kvm->srcu);
> > > > +	ret = kvm_is_visible_gfn(kvm, gfn);
> > > > +	srcu_read_unlock(&kvm->srcu, idx);
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(kvm_page_track_is_valid_gfn);
> > > This implementation is only to check whether a GFN is within a visible
> > > kvm memslot. So, why this helper function is named kvm_page_track_xxx()?
> > > Don't think it's anything related to page track, and not all of its callers
> > > in KVMGT are for page tracking.
> > 
> > KVMGT is the only user of kvm_page_track_is_valid_gfn().  kvm_is_visible_gfn()
> > has other users, just not in x86.  And long term, my goal is to allow building
> > KVM x86 without any exports.  Killing off KVM's "internal" (for vendor modules)
> > exports for select Kconfigs is easy enough, add adding a dedicated page-track API
> > solves the KVMGT angle.
> Understand!
> But personally, I don't like merging this API into page-track API as
> it obviously has nothing to do with page-track stuffs, and KVMGT also calls it for
> non-page-track purpuse.

100% agreed, but as discussed in the other patch[*], IMO the real issue is that
KVMGT is abusing KVM APIs to check the validity of GFNs that are ultimately mapped
via VFIO.  Once that issue is fixed, kvm_page_track_is_valid_gfn() can go away
entirely.  I view this as a short/medium term hack-a-fix to limit and encapsulate
KVM's API surface that is "needed" by KVMGT.

[*] https://lore.kernel.org/all/Y7cLkLUMCy+XLRwm@google.com