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Subject: Re: [virtio-dev] Re: [PATCH V2 2/2] virtio-pci: add PASID configuration extended capability
On Fri, Jan 14, 2022 at 6:38 PM Jean-Philippe Brucker <jean-philippe@linaro.org> wrote: > > On Fri, Jan 14, 2022 at 11:15:35AM +0800, Jason Wang wrote: > > > > å 2022/1/13 äå11:17, Michael S. Tsirkin åé: > > > On Thu, Jan 13, 2022 at 02:53:36PM +0000, Stefan Hajnoczi wrote: > > > > On Thu, Jan 13, 2022 at 05:45:11AM -0500, Michael S. Tsirkin wrote: > > > > > On Thu, Jan 13, 2022 at 10:32:11AM +0000, Stefan Hajnoczi wrote: > > > > > > > So my understanding is that instead of delaying the response to read > > > > > > > it's better to simply present the previous PASID value if there're any > > > > > > > pending transactions of previous PASID value. The driver can then > > > > > > > choose to poll or using timers etc. > > > > > Don't we limit these changes to before DRIVER_OK? > > > > > If we do then no previous transactions can exist. > > > > Not sure if that's possible for the vDPA virtqueue assignment use case > > > > where one large VIRTIO device provides virtqueues for many smaller vDPA > > > > devices? > > > > > > That could be the case but usually a smaller vDPA device requires a bunch of > > hardware resources more than just virtqueues. > > > > Another example is assign a queue directly to userspace with PASID for SVA. > > > > > > > > > > > > Stefan > > > Then I guess queue must be stopped? > > > > > > Yes, since we have virtqueue reset, I can limit the assigning before > > DRIVER_OK or queue(s) are reset. Does this work? > > Yes, I was wondering how this would work for the SVA use-case, where > processes can request queues from the virtio driver at runtime: > > * Process opens a fd to the queue > * virtio driver selects a disabled queue, gets a PASID for that > task and assigns it to the queue's group > * process mmaps its queue and works with it. When it is done, it closes > the fd. > * virtio driver now makes sure that the virtqueue does not issue any > more DMA with this PASID, it disables all the queues in the group by > writing 1 to their queue_reset. After reset completes, it clears the > group_pasid field. > * The driver releases the PASID (iommu_sva_unbind_device()). > Both PASID and queue can now safely be assigned to other processes. > > So making group_pasid field writeable only if all queues in the group are > disabled should work Yes, I will do that in the next version. Thanks > > Thanks, > Jean >
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