Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

[Avi Kivity <avi@scylladb.com>]

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.