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

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Date:   Mon, 13 Jul 2020 01:53:46 +0000
From:   Peng Fan <peng.fan@....com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>
CC:     "jgross@...e.com" <jgross@...e.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        dl-linux-imx <linux-imx@....com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH] xen: introduce xen_vring_use_dma

> 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?

If you could help, that would be great. You have more expertise in knowing
the whole picture.

Thanks,
Peng.