linux-kernel - Re: [PATCH] xen: introduce xen_vring_use

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Message-ID: <alpine.DEB.2.21.2007101019340.4124@sstabellini-ThinkPad-T480s>
Date:   Fri, 10 Jul 2020 10:23:22 -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

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.


If you are OK with this patch (after a little bit of clean-up), Peng,
are you OK with sending an update or do you want me to?