From: Stefano Stabellini <sstabellini@kernel.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: jgross@suse.com, Peng Fan <peng.fan@nxp.com>,
sstabellini@kernel.org, konrad.wilk@oracle.com,
jasowang@redhat.com, x86@kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
iommu@lists.linux-foundation.org, linux-imx@nxp.com,
xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] xen: introduce xen_vring_use_dma
Date: Wed, 24 Jun 2020 10:59:47 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.21.2006241047010.8121@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <20200624050355-mutt-send-email-mst@kernel.org>
On Wed, 24 Jun 2020, Michael S. Tsirkin wrote:
> On Wed, Jun 24, 2020 at 05:17:32PM +0800, Peng Fan wrote:
> > Export xen_swiotlb for all platforms using xen swiotlb
> >
> > Use xen_swiotlb to determine when vring should use dma APIs to map the
> > ring: when xen_swiotlb is enabled the dma API is required. When it is
> > disabled, it is not required.
> >
> > Signed-off-by: Peng Fan <peng.fan@nxp.com>
>
> Isn't there some way to use VIRTIO_F_IOMMU_PLATFORM for this?
> Xen was there first, but everyone else is using that now.
Unfortunately it is complicated and it is not related to
VIRTIO_F_IOMMU_PLATFORM :-(
The Xen subsystem in Linux uses dma_ops via swiotlb_xen to translate
foreign mappings (memory coming from other VMs) to physical addresses.
On x86, it also uses dma_ops to translate Linux's idea of a physical
address into a real physical address (this is unneeded on ARM.)
So regardless of VIRTIO_F_IOMMU_PLATFORM, dma_ops should be used on Xen/x86
always and on Xen/ARM if Linux is Dom0 (because it has foreign
mappings.) That is why we have the if (xen_domain) return true; in
vring_use_dma_api.
You might have noticed that I missed one possible case above: Xen/ARM
DomU :-)
Xen/ARM domUs don't need swiotlb_xen, it is not even initialized. So if
(xen_domain) return true; would give the wrong answer in that case.
Linux would end up calling the "normal" dma_ops, not swiotlb-xen, and
the "normal" dma_ops fail.
The solution I suggested was to make the check in vring_use_dma_api more
flexible by returning true if the swiotlb_xen is supposed to be used,
not in general for all Xen domains, because that is what the check was
really meant to do.
In this regards I have two more comments:
- the comment on top of the check in vring_use_dma_api is still valid
- the patch looks broken on x86: it should always return true, but it
returns false instead
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index a2de775801af..768afd79f67a 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -252,7 +252,7 @@ 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_vring_use_dma())
> > return true;
> >
> > return false;
>
>
> The comment above this should probably be fixed.
>
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu
next prev parent reply other threads:[~2020-06-24 17:59 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-24 9:17 [PATCH] xen: introduce xen_vring_use_dma Peng Fan
2020-06-24 9:06 ` Michael S. Tsirkin
2020-06-24 17:59 ` Stefano Stabellini [this message]
2020-06-24 20:47 ` Michael S. Tsirkin
2020-06-24 21:53 ` Stefano Stabellini
2020-06-24 22:16 ` Michael S. Tsirkin
2020-06-25 17:31 ` Stefano Stabellini
2020-06-26 15:32 ` Michael S. Tsirkin
2020-06-29 3:05 ` Peng Fan
2020-06-29 6:21 ` Michael S. Tsirkin
2020-06-29 6:25 ` Peng Fan
2020-06-29 6:33 ` Michael S. Tsirkin
2020-06-29 6:35 ` Peng Fan
2020-06-29 23:49 ` Stefano Stabellini
2020-06-30 1:40 ` Peng Fan
2020-06-29 23:46 ` Stefano Stabellini
2020-07-01 13:34 ` Christoph Hellwig
2020-07-01 17:34 ` Stefano Stabellini
2020-07-01 20:47 ` Michael S. Tsirkin
2020-07-01 21:23 ` Michael S. Tsirkin
2020-07-10 17:23 ` Stefano Stabellini
2020-07-11 18:44 ` Michael S. Tsirkin
2020-07-15 17:06 ` Stefano Stabellini
2020-07-13 1:53 ` Peng Fan
2020-06-29 3:00 ` Peng Fan
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.DEB.2.21.2006241047010.8121@sstabellini-ThinkPad-T480s \
--to=sstabellini@kernel.org \
--cc=boris.ostrovsky@oracle.com \
--cc=iommu@lists.linux-foundation.org \
--cc=jasowang@redhat.com \
--cc=jgross@suse.com \
--cc=konrad.wilk@oracle.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-imx@nxp.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mst@redhat.com \
--cc=peng.fan@nxp.com \
--cc=virtualization@lists.linux-foundation.org \
--cc=x86@kernel.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).