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Date: Wed, 15 Jul 2020 10:06:00 -0700 (PDT) From: Stefano Stabellini <sstabellini@...nel.org> To: "Michael S. Tsirkin" <mst@...hat.com> cc: Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com, Peng Fan <peng.fan@....com>, konrad.wilk@...cle.com, jasowang@...hat.com, x86@...nel.org, linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, Christoph Hellwig <hch@...radead.org>, iommu@...ts.linux-foundation.org, linux-imx@....com, xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com, linux-arm-kernel@...ts.infradead.org Subject: Re: [PATCH] xen: introduce xen_vring_use_dma On Sat, 11 Jul 2020, Michael S. Tsirkin wrote: > On Fri, Jul 10, 2020 at 10:23:22AM -0700, Stefano Stabellini wrote: > > Sorry for the late reply -- a couple of conferences kept me busy. > > > > > > On Wed, 1 Jul 2020, Michael S. Tsirkin wrote: > > > On Wed, Jul 01, 2020 at 10:34:53AM -0700, Stefano Stabellini wrote: > > > > Would you be in favor of a more flexible check along the lines of the > > > > one proposed in the patch that started this thread: > > > > > > > > if (xen_vring_use_dma()) > > > > return true; > > > > > > > > > > > > xen_vring_use_dma would be implemented so that it returns true when > > > > xen_swiotlb is required and false otherwise. > > > > > > Just to stress - with a patch like this virtio can *still* use DMA API > > > if PLATFORM_ACCESS is set. So if DMA API is broken on some platforms > > > as you seem to be saying, you guys should fix it before doing something > > > like this.. > > > > Yes, DMA API is broken with some interfaces (specifically: rpmesg and > > trusty), but for them PLATFORM_ACCESS is never set. That is why the > > errors weren't reported before. Xen special case aside, there is no > > problem under normal circumstances. > > So why not fix DMA API? Then this patch is not needed. It looks like the conversation is going in circles :-) I tried to explain the reason why, even if we fixed the DMA API to work with rpmesg and trusty, we still need this patch with the following email: https://marc.info/?l=linux-kernel&m=159347446709625&w=2 At that time it looked like you agreed that we needed to improve this check? (https://marc.info/?l=linux-kernel&m=159363662418250&w=2)
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