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

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Message-ID: <9a0e63b7d45beca7b7a30debd3831f433626e5f6.camel@infradead.org>
Date: Wed, 10 Jul 2024 11:20:13 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Patrick Roy <roypat@...zon.co.uk>, seanjc@...gle.com,
 pbonzini@...hat.com,  akpm@...ux-foundation.org, rppt@...nel.org,
 david@...hat.com
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 willy@...radead.org,  graf@...zon.com, derekmn@...zon.com,
 kalyazin@...zon.com, kvm@...r.kernel.org,  linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, dmatlack@...gle.com,  tabba@...gle.com,
 chao.p.peng@...ux.intel.com, xmarcalx@...zon.co.uk, James Gowans
 <jgowans@...zon.com>
Subject: Re: [RFC PATCH 3/8] kvm: pfncache: enlighten about gmem

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@...zon.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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