linux-kernel - Re: [Xen-devel] [RFC] virtio_ring: check dma_mem for xen

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Message-ID: <20190123071232.GA20526@infradead.org>
Date:   Tue, 22 Jan 2019 23:12:32 -0800
From:   "hch@...radead.org" <hch@...radead.org>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Peng Fan <peng.fan@....com>, "mst@...hat.com" <mst@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-remoteproc@...r.kernel.org" <linux-remoteproc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>, luto@...nel.org,
        jgross@...e.com, boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [RFC] virtio_ring: check dma_mem for xen_domain

On Tue, Jan 22, 2019 at 11:59:31AM -0800, Stefano Stabellini wrote:
> >  	if (!virtio_has_iommu_quirk(vdev))
> >  		return true;
> >  
> > @@ -260,7 +262,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> >  	 * the DMA API if we're a Xen guest, which at least allows
> >  	 * all of the sensible Xen configurations to work correctly.
> >  	 */
> > -	if (xen_domain())
> > +	if (xen_domain() && !dma_dev->dma_mem)
> >  		return true;
> >  
> >  	return false;
> 
> I can see you spotted a real issue, but this is not the right fix. We
> just need something a bit more flexible than xen_domain(): there are
> many kinds of Xen domains on different architectures, we basically want
> to enable this (return true from vring_use_dma_api) only when the xen
> swiotlb is meant to be used. Does the appended patch fix the issue you
> have?

The problem generally is the other way around - if dma_dev->dma_mem
is set the device decription in the device tree explicitly requires
using this memory, so we must _always_ use the DMA API.

The problem is just that that rproc driver absuses the DMA API
in horrible ways.