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

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Date:	Fri, 29 Jan 2016 10:34:59 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Andy Lutomirski <luto@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
CC:	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>,
	David Vrabel <david.vrabel@...rix.com>,
	<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 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