Re: [RFC PATCH 3/8] kvm: pfncache: enlighten about gmem

David Woodhouse
2024-07-10T10:20:25Z

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On Wed, 2024-07-10 at 10:49 +0100, Patrick Roy wrote:
> On 7/9/24 15:36, David Woodhouse wrote:

I did? It isn't September yet, surely?

> > On Tue, 2024-07-09 at 14:20 +0100, Patrick Roy wrote:
> > > KVM uses gfn_to_pfn_caches to cache translations from gfn all the way to
> > > the pfn (for example, kvm-clock caches the page storing the page used
> > > for guest/host communication this way). Unlike the gfn_to_hva_cache,
> > > where no equivalent caching semantics were possible to gmem-backed gfns
> > > (see also 858e8068a750 ("kvm: pfncache: enlighten about gmem")), here it
> > > is possible to simply cache the pfn returned by `kvm_gmem_get_pfn`.
> > >
> > > Additionally, gfn_to_pfn_caches now invalidate whenever a cached gfn's
> > > attributes are flipped from shared to private (or vice-versa).
> > >
> > > Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
> >
> > I can't see how this is safe from race conditions.
> >
> > When the GPC is invalidated from gfn_to_pfn_cache_invalidate_start()
> > its *write* lock is taken and gpc->valid is set to false.
> >
> > In parallel, any code using the GPC to access guest memory will take
> > the *read* lock, call kvm_gpc_check(), and then go ahead and use the
> > pointer to its heart's content until eventually dropping the read lock.
> >
> > Since invalidation takes the write lock, it has to wait until the GPC
> > is no longer in active use, and the pointer cannot be being
> > dereferenced.
> >
> > How does this work for the kvm_mem_is_private() check. You've added a
> > check in kvm_gpc_check(), but what if the pfn is made private
> > immediately *after* that check? Unless the code path which makes the
> > pfn private also takes the write lock, how is it safe?
>
> Ah, you're right - I did in fact overlook this. I do think that it works
> out though: kvm_vm_set_mem_attributes, which is used for flipping
> between shared/private, registers the range which had its attributes
> changed for invalidation, and thus gfn_to_pfn_cache_invalidate_start
> should get called for it (although I have to admit I do not immediately
> see what the exact callstack for this looks like, so maybe I am
> misunderstanding something about invalidation here?).

In that case, wouldn't that mean the explicit checks on gpc->is_private
matching kvm_mem_is_private() would be redundant and you can remove
them because you can trust that gpc->valid would be cleared?

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