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

["Durrant, Paul" <pdurrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 27 November 2019 15:26
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné
> <roger.pau@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] x86 / iommu: set up a scratch page in the
> quarantine domain
> 
> 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.

No. Unfortunately the h/w has zero tolerance for some faults.

  Paul

> 
> Jan
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