Re: [Xen-devel] [PATCH for-4.12 3/8] iommu/pvh: add reserved regions below 1MB to the iommu page tables

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 05, 2019 at 08:18:52AM -0700, Jan Beulich wrote:
> >>> On 05.02.19 at 15:01, <roger.pau@xxxxxxxxxx> wrote:
> > On Tue, Feb 05, 2019 at 05:49:07AM -0700, Jan Beulich wrote:
> >> >>> On 05.02.19 at 12:15, <roger.pau@xxxxxxxxxx> wrote:
> >> > On Tue, Feb 05, 2019 at 03:47:49AM -0700, Jan Beulich wrote:
> >> >> >>> On 30.01.19 at 11:36, <roger.pau@xxxxxxxxxx> wrote:
> >> > Maybe it's better to just forget about the pre-haswell workarounds and
> >> > enable the iommu before populating the p2m, that would certainly
> >> > simply the code here by removing the low 1MB special casing.
> >> 
> >> Are you convinced that those workarounds are attributable to the
> >> CPU family, and that hence with Haswell and newer they're gone
> >> altogether?
> > 
> > Not sure, I guess it's more likely part of the chipset rather the CPU
> > itself? But since chipsets are usually paired with CPU families, it's
> > quite likely the bogus chipset was only used in conjunction with
> > pre-Haswell CPUs.
> 
> I'd expect it's largely the firmware screwing things up.
> 
> > Anyway, I'm happy to change the order so that the iommu is enabled
> > before the p2m is populated and then drop this workaround from the
> > iommu code. Would you be fine with such a change?
> 
> Personally I would be, but if the implication would that PVH won't
> work on pre-Haswell anymore, then I think this can't be settled
> just between the two of us.

I no longer have that pre-Haswell system, and when I reported
this to Intel the response I got was that they where unable to
reproduce the issue, so it might be something quite specific to my
test box.

My suggestion would be to switch to the normal flow (iommu init first,
then populate p2m RAM regions) and wait for complains. ATM I have no
way to assert that the current code would boot on that broken system
anyway, and PVH dom0 is still experimental so people playing with it
should be expecting to find issues and test fixes.

Thanks, Roger.

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