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

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Date:   Wed, 23 Jan 2019 13:04:33 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     "hch@...radead.org" <hch@...radead.org>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Peng Fan <peng.fan@....com>, "mst@...hat.com" <mst@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "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, 22 Jan 2019, hch@...radead.org wrote:
> 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.

If vring_use_dma_api is actually supposed to return true when
dma_dev->dma_mem is set, then both Peng's patch and the patch I wrote
are not fixing the real issue here.

I don't know enough about remoteproc to know where the problem actually
lies though.