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[2003:cb:c719:5b00:61af:900f:3aef:3af3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4247d208e08sm17944335e9.35.2024.06.20.02.00.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Jun 2024 02:00:46 -0700 (PDT) Message-ID: <26aa1ad6-fcdd-4fe2-b763-6938555f93db@redhat.com> Date: Thu, 20 Jun 2024 11:00:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning To: Fuad Tabba Cc: John Hubbard , Elliot Berman , Andrew Morton , Shuah Khan , Matthew Wilcox , maz@kernel.org, kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, pbonzini@redhat.com, Jason Gunthorpe References: <20240618-exclusive-gup-v1-0-30472a19c5d1@quicinc.com> <7fb8cc2c-916a-43e1-9edf-23ed35e42f51@nvidia.com> <14bd145a-039f-4fb9-8598-384d6a051737@redhat.com> <489d1494-626c-40d9-89ec-4afc4cd0624b@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: axh1hs54z9rahg4oyzqw95wzt8mgcy4b X-Rspamd-Queue-Id: CE45F40017 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1718874051-704600 X-HE-Meta: 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 i8F02zZf 85Gg15rYAtejDvnn0H4OvdJPKllYNhdTsAjFq4ECqEMsTqfTDYAwg1uc8NRmmEuNkxEVqaqWK8GV7cxDhwnnyd26ZpjoRSSEPNYlpVVB/8l0VTriGOz/W63ZWdcOAdxQVOpZBX4u4W8rWKfbTctyl8+DGoUCE0djt73lD/f1jXd+p/FVTzFKzSWsbTTR//68SB8w7+pw24F0LAsZFurn3IMCxJGFB0tjz0h+GMXE+xLfou9s1BkjJFwgglWvgSI11o6GX5fgoN8kLySb1S2ePNgc1qxv7AF3wFn1MU22E+DuKx/ZdbIZdoeMk0dtWzdGdbPNNJzsZsx4CeKKIi+Suh7y3jCcjYyCkLt/oumgroz80fbFEbbZOExLN1vEFUQUh5Cx4BUT0E+dShbvN9AUo73RK70MUR9p262DTKjTAfXI94XD7NNjutzfTmxRuekGG8kjOuHf5a8N/WXg= 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: >> Yes, and I think we might have to revive that discussion, unfortunately. >> I started thinking about this, but did not reach a conclusion. Sharing >> my thoughts. >> >> The minimum we might need to make use of guest_memfd (v1 or v2 ;) ) not >> just for private memory should be: >> >> (1) Have private + shared parts backed by guest_memfd. Either the same, >> or a fd pair. >> (2) Allow to mmap only the "shared" parts. >> (3) Allow in-place conversion between "shared" and "private" parts. > > These three were covered (modulo bugs) in the guest_memfd() RFC I'd > sent a while back: > > https://lore.kernel.org/all/20240222161047.402609-1-tabba@google.com/ I remember there was a catch to it (either around mmap or pinning detection -- or around support for huge pages in the future; maybe these count as BUGs :) ). I should probably go back and revisit the whole thing, I was only CCed on some part of it back then. > >> (4) Allow migration of the "shared" parts. > > We would really like that too, if they allow us :) > >> A) Convert shared -> private? >> * Must not be GUP-pinned >> * Must not be mapped >> * Must not reside on ZONE_MOVABLE/MIGRATE_CMA >> * (must rule out any other problematic folio references that could >> read/write memory, might be feasible for guest_memfd) >> >> B) Convert private -> shared? >> * Nothing to consider >> >> C) Map something? >> * Must not be private > > A,B and C were covered (again, modulo bugs) in the RFC. > >> For ordinary (small) pages, that might be feasible. >> (ZONE_MOVABLE/MIGRATE_CMA might be feasible, but maybe we could just not >> support them initially) >> >> The real fun begins once we want to support huge pages/large folios and >> can end up having a mixture of "private" and "shared" per huge page. But >> really, that's what we want in the end I think. > > I agree. > >> Unless we can teach the VM to not convert arbitrary physical memory >> ranges on a 4k basis to a mixture of private/shared ... but I've been >> told we don't want that. Hm. >> >> >> There are two big problems with that that I can see: >> >> 1) References/GUP-pins are per folio >> >> What if some shared part of the folio is pinned but another shared part >> that we want to convert to private is not? Core-mm will not provide the >> answer to that: the folio maybe pinned, that's it. *Disallowing* at >> least long-term GUP-pins might be an option. > > Right. > >> To get stuff into an IOMMU, maybe a per-fd interface could work, and >> guest_memfd would track itself which parts are currently "handed out", >> and with which "semantics" (shared vs. private). >> >> [IOMMU + private parts might require that either way? Because, if we >> dissallow mmap, how should that ever work with an IOMMU otherwise]. > > Not sure if IOMMU + private makes that much sense really, but I think > I might not really understand what you mean by this. A device might be able to access private memory. In the TDX world, this would mean that a device "speaks" encrypted memory. At the same time, a device might be able to access shared memory. Maybe devices can do both? What do do when converting between private and shared? I think it depends on various factors (e.g., device capabilities). [...] >> I recall quite some details with memory renting or so on pKVM ... and I >> have to refresh my memory on that. > > I really would like to get to a place where we could investigate and > sort out all of these issues. It would be good to know though, what, > in principle (and not due to any technical limitations), we might be > allowed to do and expand guest_memfd() to do, and what out of > principle is off the table. As Jason said, maybe we need a revised model that can handle [...] private+shared properly. -- Cheers, David / dhildenb