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[2003:cb:c708:e100:4f41:ff29:a59f:8c7a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436e9e6251asm55521085e9.40.2025.01.10.05.45.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Jan 2025 05:45:40 -0800 (PST) Message-ID: <17db435a-8eca-4132-8481-34a6b0e986cb@redhat.com> Date: Fri, 10 Jan 2025 14:45:39 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/7] Enable shared device assignment To: Jason Gunthorpe Cc: Chenyi Qiang , Alexey Kardashevskiy , Paolo Bonzini , Peter Xu , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= , Michael Roth , qemu-devel@nongnu.org, kvm@vger.kernel.org, Williams Dan J , Peng Chao P , Gao Chao , Xu Yilun References: <20241213070852.106092-1-chenyi.qiang@intel.com> <2737cca7-ef2d-4e73-8b5a-67698c835e77@amd.com> <8457e035-40b0-4268-866e-baa737b6be27@intel.com> <6ac5ddea-42d8-40f2-beec-be490f6f289c@amd.com> <8f953ffc-6408-4546-a439-d11354b26665@intel.com> <57a3869d-f3d1-4125-aaa5-e529fb659421@intel.com> <008bfbf2-3ea4-4e6c-ad0d-91655cdfc4e8@amd.com> <1361f0b4-ddf8-4a83-ba21-b68321d921da@intel.com> <20250110132021.GE5556@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.01.25 14:20, Jason Gunthorpe wrote: Thanks for your reply, I knew CCing you would be very helpful :) > On Fri, Jan 10, 2025 at 09:26:02AM +0100, David Hildenbrand wrote: >>>>>>>>>>> One limitation (also discussed in the guest_memfd >>>>>>>>>>> meeting) is that VFIO expects the DMA mapping for >>>>>>>>>>> a specific IOVA to be mapped and unmapped with the >>>>>>>>>>> same granularity. > > Not just same granularity, whatever you map you have to unmap in > whole. map/unmap must be perfectly paired by userspace. Right, that's what virtio-mem ends up doing by mapping each memory block (e.g., 2 MiB) separately that could be unmapped separately. It adds "overhead", but at least you don't run into "no, you cannot split this region because you would be out of memory/slots" or in the past issues with concurrent ongoing DMA. > >>>>>>>>>>> such as converting a small region within a larger >>>>>>>>>>> region. To prevent such invalid cases, all >>>>>>>>>>> operations are performed with 4K granularity. The >>>>>>>>>>> possible solutions we can think of are either to >>>>>>>>>>> enable VFIO to support partial unmap > > Yes, you can do that, but it is aweful for performance everywhere Absolutely. In your commit I read: "Implement the cut operation to be hitless, changes to the page table during cutting must cause zero disruption to any ongoing DMA. This is the expectation of the VFIO type 1 uAPI. Hitless requires HW support, it is incompatible with HW requiring break-before-make." So I guess that would mean that, depending on HW support, one could avoid disabling large pages to still allow for atomic cuts / partial unmaps that don't affect concurrent DMA. What would be your suggestion here to avoid the "map each 4k page individually so we can unmap it individually" ? I didn't completely grasp that, sorry. From "IIRC you can only trigger split using the VFIO type 1 legacy API. We would need to formalize split as an IOMMUFD native ioctl. Nobody should use this stuf through the legacy type 1 API!!!!" I assume you mean that we can only avoid the 4k map/unmap if we add proper support to IOMMUFD native ioctl, and not try making it fly somehow with the legacy type 1 API? -- Cheers, David / dhildenb