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 12:27:39 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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 12:18 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> 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.
>

In that case, let me test for real.  Can you point me to a git tree
when you have patches 1-8 staged and I'll spin patch 9 v2 and test it
with the real context?

--Andy