linux-kernel - Re: [Xen-devel] [PATCH v5 09/10] vring: Use the DMA API on Xen

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Date:	Sun, 31 Jan 2016 22:18:17 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	Andy Lutomirski <luto@...nel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Joerg Roedel <jroedel@...e.de>, KVM <kvm@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	sparclinux@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"David S. Miller" <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [Xen-devel] [PATCH v5 09/10] vring: Use the DMA API on Xen

On Sun, Jan 31, 2016 at 12:13:58PM -0800, Andy Lutomirski wrote:
> On Sun, Jan 31, 2016 at 12:09 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > On Fri, Jan 29, 2016 at 10:34:59AM +0000, David Vrabel wrote:
> >> On 29/01/16 02:31, Andy Lutomirski wrote:
> >> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> >> > ---
> >> >  drivers/virtio/virtio_ring.c | 12 ++++++++++++
> >> >  1 file changed, 12 insertions(+)
> >> >
> >> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> >> > index c169c6444637..305c05cc249a 100644
> >> > --- a/drivers/virtio/virtio_ring.c
> >> > +++ b/drivers/virtio/virtio_ring.c
> >> > @@ -47,6 +47,18 @@
> >> >
> >> >  static bool vring_use_dma_api(void)
> >> >  {
> >> > +#if defined(CONFIG_X86) && defined(CONFIG_XEN)
> >> > +   /*
> >> > +    * In theory, it's possible to have a buggy QEMU-supposed
> >> > +    * emulated Q35 IOMMU and Xen enabled at the same time.  On
> >> > +    * such a configuration, virtio has never worked and will
> >> > +    * not work without an even larger kludge.  Instead, enable
> >> > +    * the DMA API if we're a Xen guest, which at least allows
> >> > +    * all of the sensible Xen configurations to work correctly.
> >> > +    */
> >> > +   return static_cpu_has(X86_FEATURE_XENPV);
> >>
> >> You want:
> >>
> >>     if (xen_domain())
> >>         return true;
> >>
> >> Without the #if so we use the DMA API for all types of Xen guest on all
> >> architectures.
> >>
> >> David
> >
> > I doubt HVM domains can have virtio devices.
> >
> 
> They certainly can under nested virt (L0 provides virtio device, L1 is
> Xen, and L2 is Linux).  Of course, this won't work given the current
> QEMU situation unless Xen can pass things through to dom0 without an
> IOMMU, which seems plausible to me.
> 
> But yes, xen_domain() sounds right to me.  I just failed to find that
> function when I wrote this patch.
> 
> Michael, if you like the rest of the series, I'd be okay if you
> changed this patch to use xen_domain() when you apply it.  If I send a
> v2, I'll fix it up.
> 
> --Andy

I'd rather you just posted a tested v2 of 9/10 for now as I don't test
Xen.  It seems easy but I had more than my share of obvious fixes
failing spectacularly.

-- 
MST