From: "Michael S. Tsirkin" <mst@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Eric Auger <eric.auger@redhat.com>,
QEMU Developers <qemu-devel@nongnu.org>,
Stefan Hajnoczi <stefanha@redhat.com>
Subject: Re: why is iommu_platform set to off by default?
Date: Tue, 30 Nov 2021 18:44:01 -0500 [thread overview]
Message-ID: <20211130184126-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CAFEAcA-Fb15x7hAe-g8hP8HL1xB14iGnYi5=ZJM=0G0_hbBjTw@mail.gmail.com>
On Tue, Nov 30, 2021 at 02:32:49PM +0000, Peter Maydell wrote:
> I've just spent a day or so trying to track down why PCI passthrough
> of a virtio-blk-pci device wasn't working. The problem turns out to be
> that by default virtio pci devices don't use the IOMMU, even when the
> machine model has created an IOMMU and arranged for the PCI bus to
> be underneath it. So when the L2 guest tries to program the virtio device,
> the virtio device treats the IPAs it writes as if they were PAs and
> of course the data structures it's looking for aren't there.
Because this is what legacy guests expect, and legacy configs are
much more common than nested.
> Why do we default this to 'off'? It seems pretty unhelpful not to
> honour the existence of the IOMMU, and the failure mode is pretty
> opaque (L2 guest just hangs)...
>
> thanks
> -- PMM
This should be handled by VFIO in L1 really, it can check for a
device quirk and refuse binding if the feature bit
is disabled.
--
MST
prev parent reply other threads:[~2021-11-30 23:44 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-30 14:32 why is iommu_platform set to off by default? Peter Maydell
2021-11-30 16:38 ` Stefan Hajnoczi
2021-12-01 0:00 ` Michael S. Tsirkin
2021-11-30 23:44 ` Michael S. Tsirkin [this message]
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