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

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

> -----Original Message-----
> From: Jürgen Groß <jgross@xxxxxxxx>
> Sent: 10 December 2019 09:07
> To: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; Durrant, Paul
> <pdurrant@xxxxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; Roger Pau Monné <roger.pau@xxxxxxxxxx>; Wei
> Liu <wl@xxxxxxx>
> Subject: Re: [PATCH v2] x86 / iommu: set up a scratch page in the
> quarantine domain
> 
> On 10.12.19 09:57, Jan Beulich wrote:
> > On 10.12.2019 09:12, Jürgen Groß wrote:
> >> On 10.12.19 09:05, Jan Beulich wrote:
> >>> On 10.12.2019 08:16, Tian, Kevin wrote:
> >>>> While the quarantine idea sounds good overall, I'm still not
> convinced
> >>>> to have it the only way in place just for handling some known-buggy
> >>>> device. It kills the possibility of identifying a new buggy device
> and then
> >>>> deciding not to use it in the first space... I thought about whether
> it
> >>>> will get better when future IOMMU implements A/D bit - by checking
> >>>> access bit being set then we'll know some buggy device exists, but,
> >>>> the scratch page is shared by all devices then we cannot rely on this
> >>>> feature to find out the actual buggy one.
> >>>
> >>> Thinking about it - yes, I think I agree. This (as with so many
> >>> workarounds) would better be an off-by-default one. The main issue
> >>> I understand this would have is that buggy systems then might hang
> >>> without even having managed to get a log message out - Paul?
> >>>
> >>> Jürgen - would you be amenable to an almost last minute refinement
> >>> here (would then also need to still be backported to 4.12.2, or
> >>> the original backport reverted, to avoid giving the impression of
> >>> a regression)?
> >>
> >> So what is your suggestion here? To have a boot option (defaulting to
> >> off) for enabling the scratch page?
> >
> > Yes (and despite having seen Paul's reply).
> 
> I'd release ack such a patch in case you come to an agreement regarding
> the default soon.
> 

Ok. The default is not that crucial. Perhaps it's just me who thinks defaults 
should be chosen on the basis of being most likely to result in a working 
system.

  Paul

> 
> Juergen
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