Re: [Xen-devel] [PATCH] arm/acpi: hide watchdog timer in GTDT table for dom0

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

On Tue, Nov 29, 2016 at 03:40:37AM -0700, Jan Beulich wrote:
> >>> On 29.11.16 at 03:59, <shankerd@xxxxxxxxxxxxxx> wrote:
> > Either we have to hide the watchdog timer section in GTDT or emulate
> > watchdog timer block for dom0. Otherwise, system gets panic when
> > dom0 accesses its MMIO registers. The current XEN doesn't support
> > virtualization of watchdog timer, so hide the watchdog timer section
> > for dom0.
> > 
> > Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
> 
> The mere need for a patch like this is, to me at least, a pretty
> clear indication that such black listing models don't work well
> (and using white listing instead would likely be too restrictive).
> As the same is about to become an issue on x86 too (with
> PVHv2) I think there's a need to reconsider how to deal with
> ACPI with other than a traditional PV Dom0, namely whether
> (a) it wouldn't make sense to expect a little more awareness
> by the Dom0 kernel, and
> (b) whether we shouldn't unconditionally hand everything to
> Dom0 which Xen doesn't explicitly make use of itself (implying
> that MMIO regions get suitably prepared either during boot
> or on demand).

I cannot speak about ARM, but given that support for ACPI was added in the 
previous release, and is AFAIK still marked as experimental, I expect such 
fixes 
to be normal.

Then, speaking about the general picture and approach, (a) is mostly true on 
x86 
PVHv2 Dom0, while I don't see the point in also doing (b). Regarding (a), it's 
quite clear that some awareness by Dom0 is going to be needed when running as 
Dom0 (report MCFG regions and the poweroff sequence at least) as much as I 
would like to avoid that. Regarding (b), there are tables that we already know 
are completely useless to Dom0 ATM, like DMAR, SRAT, SLIT... IMHO, those should 
not be presented to Dom0 at all, because if they are presented now they should 
be ignored by Dom0, and if we ever want to somehow for example report NUMA 
topology to Dom0, we would be screwed.

Roger.

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