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Message-ID: <ZFEqTo+l/S8IkBQm@nvidia.com> Date: Tue, 2 May 2023 12:20:46 -0300 From: Jason Gunthorpe <jgg@...dia.com> To: Matthew Rosato <mjrosato@...ux.ibm.com> Cc: David Hildenbrand <david@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>, Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>, Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Bjorn Topel <bjorn@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jonathan Lemon <jonathan.lemon@...il.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christian Brauner <brauner@...nel.org>, Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>, John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>, "Kirill A . Shutemov" <kirill@...temov.name>, Pavel Begunkov <asml.silence@...il.com>, Mika Penttila <mpenttil@...hat.com>, Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com> Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings On Tue, May 02, 2023 at 10:54:35AM -0400, Matthew Rosato wrote: > On 5/2/23 10:15 AM, David Hildenbrand wrote: > > On 02.05.23 16:04, Jason Gunthorpe wrote: > >> On Tue, May 02, 2023 at 03:57:30PM +0200, David Hildenbrand wrote: > >>> On 02.05.23 15:50, Jason Gunthorpe wrote: > >>>> On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote: > >>>>>> Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT. > >>>>> > >>>>> Okay, so IIRC we'll fail starting the domain early, that's good. And if we > >>>>> pin all guest memory (instead of small pieces dynamically), there is little > >>>>> existing use for file-backed RAM in such zPCI configurations (because memory > >>>>> cannot be reclaimed either way if it's all pinned), so likely there are no > >>>>> real existing users. > >>>> > >>>> Right, this is VFIO, the physical HW can't tolerate not having pinned > >>>> memory, so something somewhere is always pinning it. > >>>> > >>>> Which, again, makes it weird/wrong that this KVM code is pinning it > >>>> again :\ > >>> > >>> IIUC, that pinning is not for ordinary IOMMU / KVM memory access. It's for > >>> passthrough of (adapter) interrupts. > >>> > >>> I have to speculate, but I guess for hardware to forward interrupts to the > >>> VM, it has to pin the special guest memory page that will receive the > >>> indications, to then configure (interrupt) hardware to target the interrupt > >>> indications to that special guest page (using a host physical address). > >> > >> Either the emulated access is "CPU" based happening through the KVM > >> page table so it should use mmu_notifier locking. > >> > >> Or it is "DMA" and should go through an IOVA through iommufd pinning > >> and locking. > >> > >> There is no other ground, nothing in KVM should be inventing its own > >> access methodology. > > > > I might be wrong, but this seems to be a bit different. > > > > It cannot tolerate page faults (needs a host physical address), so > > memory notifiers don't really apply. (as a side note, KVM on s390x > > does not use mmu notifiers as we know them) > > The host physical address is one shared between underlying firmware > and the host kvm. Either might make changes to the referenced page > and then issue an alert to the guest via a mechanism called GISA, > giving impetus to the guest to look at that page and process the > event. As you say, firmware can't tolerate the page being > unavailable; it's expecting that once we feed it that location it's > always available until we remove it (kvm_s390_pci_aif_disable). That is a CPU access delegated to the FW without any locking scheme to make it safe with KVM :\ It would have been better if FW could inject it through the kvm page tables so it has some coherency. Otherwise you have to call this "DMA", I think. How does s390 avoid mmu notifiers without having lots of problems?? It is not really optional to hook the invalidations if you need to build a shadow page table.. Jason
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