Re: [Xen-devel] [PATCH v2] x86 / iommu: set up a scratch page in the quarantine domain

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.11.2019 18:11, Paul Durrant wrote:
> This patch introduces a new iommu_op to facilitate a per-implementation
> quarantine set up, and then further code for x86 implementations
> (amd and vtd) to set up a read-only scratch page to serve as the source
> for DMA reads whilst a device is assigned to dom_io. DMA writes will
> continue to fault as before.
> 
> The reason for doing this is that some hardware may continue to re-try
> DMA (despite FLR) in the event of an error, or even BME being cleared, and
> will fail to deal with DMA read faults gracefully. Having a scratch page
> mapped will allow pending DMA reads to complete and thus such buggy
> hardware will eventually be quiesced.
> 
> NOTE: These modifications are restricted to x86 implementations only as
>       the buggy h/w I am aware of is only used with Xen in an x86
>       environment. ARM may require similar code but, since I am not
>       aware of the need, this patch does not modify any ARM implementation.
> 
> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> There is still the open question of whether use of a scratch page ought
> to be gated on something, either are run-time or compile-time.

I have no clear opinion either way here. The workaround seems low
overhead enough that there may not be a need to have an admin (or
build time) control for this.

As to 4.13: The quarantining as a whole is pretty fresh. While it
has been backported to security maintained trees, I'd still consider
it a new feature in 4.13, and hence this workaround at least eligible
for consideration.

Jan

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