Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support
From: Avi Kivity
Date: Thu Oct 08 2015 - 05:44:23 EST
On 10/08/2015 12:16 PM, Michael S. Tsirkin wrote:
On Thu, Oct 08, 2015 at 11:46:30AM +0300, Avi Kivity wrote:
On 10/08/2015 10:32 AM, Michael S. Tsirkin wrote:
On Thu, Oct 08, 2015 at 08:33:45AM +0300, Avi Kivity wrote:
It is good practice to defend against root oopsing the kernel, but in some
cases it cannot be achieved.
Absolutely. That's one of the issues with these patches. They don't even
try where it's absolutely possible.
Are you referring to blocking the maps of the msix BAR areas?
For example. There are more. I listed some of the issues on the mailing
list, and I might have missed some. VFIO has code to address all this,
people should share code to avoid duplication, or at least read it
to understand the issues.
All but one of those are unrelated to the patch that adds msix support.
I think there is value in that. The value is small because a
corruption is more likely in the dynamic memory responsible for tens
of millions of DMA operations per second, rather than a static 4K
area, but it exists.
There are other bugs which will hurt e.g. each time application does not
exit gracefully.
uio_pci_generic disables DMA when the device is removed, so we're safe
here, at least if files are released before the address space.
But well, heh :) That's precisely my feeling about the whole "running
userspace drivers without an IOMMU" project. The value is small
since modern hardware has fast IOMMUs, but it exists.
For users that don't have iommus at all (usually because it is taken by
the hypervisor), it has great value.
I can't comment on iommu overhead; for my use case it is likely
negligible and we will use an iommu when available; but apparently it
matters for others.
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