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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.11.2019 12:32, Jürgen Groß wrote:
> On 28.11.19 12:17, Jan Beulich wrote:
>> 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.
> 
> I agree.
> 
> Release-acked-by: Juergen Gross <jgross@xxxxxxxx>

I notice this has been committed meanwhile. I had specifically not
done so due to the still missing VT-d ack, seeing that this wasn't
an entirely "trivial" change.

Jan

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