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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.11.2019 16:18,  Durrant, Paul  wrote:
>> -----Original Message-----
>> From: Tian, Kevin <kevin.tian@xxxxxxxxx>
>> Sent: 25 November 2019 08:22
>> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>; Andrew Cooper
>> <andrew.cooper3@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau Monné
>> <roger.pau@xxxxxxxxxx>
>> Subject: RE: [PATCH] x86 / iommu: set up a scratch page in the quarantine
>> domain
>>
>>> From: Paul Durrant [mailto:pdurrant@xxxxxxxxxx]
>>> Sent: Wednesday, November 20, 2019 8:09 PM
>>>
>>> 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/wrote scratch page to serve as the
>> source/
>>> target for all DMA whilst a device is assigned to dom_io.
>>>
>>> The reason for doing this is that some hardware may continue to re-try
>>> DMA, despite FLR, in the event of an error. Having a scratch page mapped
>>> will allow pending DMA to drain and thus quiesce such buggy hardware.
>>
>> then there is no diagnostics at all since all faults are quiescent now...
>> why do we want to support such buggy hardware? Is it better to make
>> it an default-off option since buggy is supposed to niche case?
> 
> I guess it could be a command line option... perhaps making the new
> 'iommu=quarantine' boolean into something more complex, but I'm not
> sure it's really worth it. Perhaps a compile time option would be
> better?

Yet another option: How about installing the scratch page mappings
only after a (handful of) IOMMU faults? But of course there was the
related earlier question of whether indeed our turning off of bus
mastering doesn't already help silencing the faults.

Jan

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