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[Xen-devel] PVH and ACPI discussion


While looking at some tangential issues I realized that the 'VGA Not
Present' flag that Xen currently sets for PVH DomUs might be slightly
different from what we expect it to mean. The purpose was that Xen
would set this flag to denote there's no VGA MMIO region in the low
1MB, so that the guest OS would not reserve memory in that area
thinking there's a MMIO window there. The memory map provided to a PVH
DomU typically contains a single RAM range that expands from 0 to the
selected amount of memory.

The description of such flag by the ACPI spec (6.2A) however is as

"If set, indicates to OSPM that it must not blindly probe the VGA
hardware (that responds to MMIO addresses A0000h-BFFFFh and IO ports
3B0h-3BBh and 3C0h-3DFh) that may cause machine check on this system.
If clear, indicates to OSPM that it is safe to probe the VGA

My reading of the above text would make me think that if the flag is
set the memory region A0000h-BFFFFh should not be used at all, and
that would be in conflict with the memory map that's provided to
guests (which lists this area as RAM).

I'm not convinced of the best way to proceed here. I can contact the
ACPI working group and try to clarify the meaning of the flag, or
inquiry if there's a more suitable flag for or use case, but I would
like to hear others opinion on this topic.

Secondly, I've also been looking at whether it would make sense to set
the ACPI reduced hardware flag for PVH DomUs, according to the
description in the ACPI spec:

"For certain classes of systems the ACPI Hardware Specification may
not be adequate. Examples include legacy-free, UEFI-based platforms
with recent processors, and those implementing mobile platform
architectures. For such platforms, a Hardware-reduced ACPI mode is

Certainly the legacy-free and UEFI part is quite applicable to PVH
DomU, for which we don't plan to support legacy BIOS and only provide
UEFI firmware in the long term.

Reduced HW ACPI also gets rid of the SCI interrupt, and instead
provides some other methods to signal ACPI events (note we don't
use any ACPI event ATM for PVH DomU). It also gets rid of a bunch of
FADT fields that we don't use for PVH DomU either.

I however seem to remember some past discussion about PVH DomU and
reduced ACPI, but I cannot find the thread. If there are no objections
I think we should look into this (likely discuss with the ACPI working
group) in order to figure out if reduced HW ACPI could work for us,
and how the event delivery could be implemented for PVH DomU if it
turns out we need it later on. It might make sense to also figure out
what other people do, like HyperV Gen2 instances (which also get rid
of a lot of legacy hw).

Thanks, Roger.

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