Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

From: Andy Lutomirski <luto@amacapital.net>
To: Rusty Russell <rusty@rustcorp.com.au>,
	"Michael S. Tsirkin" <mst@redhat.com>
Cc: "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
	xen-devel <Xen-devel@lists.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"linux390@de.ibm.com" <linux390@de.ibm.com>,
	Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API
Date: Mon, 27 Jul 2015 18:08:59 -0700	[thread overview]
Message-ID: <CALCETrU2o27udqjvREDFyoNtKtaKvuHTyYRaK+Qt_1Q3tPWYDQ@mail.gmail.com> (raw)

On Mon, Sep 1, 2014 at 10:39 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> This fixes virtio on Xen guests as well as on any other platform
> that uses virtio_pci on which physical addresses don't match bus
> addresses.
>
> This can be tested with:
>
>     virtme-run --xen xen --kimg arch/x86/boot/bzImage --console
>
> using virtme from here:
>
>     https://git.kernel.org/cgit/utils/kernel/virtme/virtme.git
>
> Without these patches, the guest hangs forever.  With these patches,
> everything works.
>

Dusting off an ancient thread.

Now that the dust has accumulated^Wsettled, is it worth pursuing this?
 I think the situation is considerably worse than it was when I
originally wrote these patches: I think that QEMU now supports a nasty
mode in which the guest's PCI bus appears to be behind an IOMMU but
the virtio devices on that bus punch straight through that IOMMU.

I have a half-hearted port to modern kernels here:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=virtio_ring_xen

I didn't implement DMA API access for virtio_pci_modern, and I have no
idea what to do about detecting whether a given virtio device honors
its IOMMU or not.

--Andy