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Wed, 10 Jul 2024 10:46:52 +0000 Received: from EX19MTAUWB001.ant.amazon.com (10.250.64.248) by EX19D020UWC004.ant.amazon.com (10.13.138.149) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.1258.34; Wed, 10 Jul 2024 10:46:51 +0000 Received: from [127.0.0.1] (172.19.88.180) by mail-relay.amazon.com (10.250.64.254) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.1258.34 via Frontend Transport; Wed, 10 Jul 2024 10:46:46 +0000 Message-ID: Date: Wed, 10 Jul 2024 11:46:45 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird To: David Woodhouse , , , , , CC: , , , , , , , , , , , , , , , , , James Gowans References: <20240709132041.3625501-1-roypat@amazon.co.uk> <20240709132041.3625501-4-roypat@amazon.co.uk> <9a0e63b7d45beca7b7a30debd3831f433626e5f6.camel@infradead.org> From: Patrick Roy Content-Language: en-US Autocrypt: addr=roypat@amazon.co.uk; keydata= xjMEY0UgYhYJKwYBBAHaRw8BAQdA7lj+ADr5b96qBcdINFVJSOg8RGtKthL5x77F2ABMh4PN NVBhdHJpY2sgUm95IChHaXRodWIga2V5IGFtYXpvbikgPHJveXBhdEBhbWF6b24uY28udWs+ wpMEExYKADsWIQQ5DAcjaM+IvmZPLohVg4tqeAbEAgUCY0UgYgIbAwULCQgHAgIiAgYVCgkI CwIEFgIDAQIeBwIXgAAKCRBVg4tqeAbEAmQKAQC1jMl/KT9pQHEdALF7SA1iJ9tpA5ppl1J9 AOIP7Nr9SwD/fvIWkq0QDnq69eK7HqW14CA7AToCF6NBqZ8r7ksi+QLOOARjRSBiEgorBgEE AZdVAQUBAQdAqoMhGmiXJ3DMGeXrlaDA+v/aF/ah7ARbFV4ukHyz+CkDAQgHwngEGBYKACAW IQQ5DAcjaM+IvmZPLohVg4tqeAbEAgUCY0UgYgIbDAAKCRBVg4tqeAbEAtjHAQDkh5jZRIsZ 7JMNkPMSCd5PuSy0/Gdx8LGgsxxPMZwePgEAn5Tnh4fVbf00esnoK588bYQgJBioXtuXhtom 8hlxFQM= In-Reply-To: <9a0e63b7d45beca7b7a30debd3831f433626e5f6.camel@infradead.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Stat-Signature: 6dpcs9u5mfismzsbnptwgz8fonzstdgj X-Rspamd-Queue-Id: C101C4001C X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1720608419-637190 X-HE-Meta: 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 bdugxb/M zbRUl/mNSXPtRGIPzGf7q8qTTsYVRTZoax8+Si0jBZ0BXt0RN+pqNwQgL3G3oqqG30oAMOfAfB6hvMnhQ5PlKNcvZyE1G4Sth1RMjq3nYOmmTMRwVIQdbU0hJc0JU0uWL1/6pPMmSi2ebZ/rdRWTpnz0zzI0avmI6u7zN+2RXUqU4po/VeFMn3lknpEtx/Fxn/XXYLRfmR02BdmiKdP3x2Kxa9x9cfvwVssmciBim5tW3HjQ= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 2024-07-10 at 11:20 +0100, David Woodhouse wrote: > 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? Argh, thanks for letting me know, I think I've whacked some sense into my mail client now :) >>> 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 >>> >>> 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? > Right, yes, it would indeed mean that. I'll double-check my assumption about the whole invalidation thing and adjust the code for the next iteration!