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Re: [PATCH] revert "hvmloader: PA range 0xfc000000-0xffffffff should be UC"
- To: Andrew Cooper <amc96@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 16 Dec 2021 12:09:17 +0100
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Thu, 16 Dec 2021 11:09:37 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Thu, Dec 16, 2021 at 11:00:17AM +0000, Andrew Cooper wrote:
> On 16/12/2021 09:54, Roger Pau Monné wrote:
> > On Tue, Dec 14, 2021 at 09:21:22AM +0100, Jan Beulich wrote:
> >> This reverts commit c22bd567ce22f6ad9bd93318ad0d7fd1c2eadb0d.
> >>
> >> While its description is correct from an abstract or real hardware pov,
> >> the range is special inside HVM guests. The range being UC in particular
> >> gets in the way of OVMF, which places itself at [FFE00000,FFFFFFFF].
> > I would assume this range to be unpopulated? Does hvmloader populate
> > it in order to place ovmf?
>
> It's generally not unpopulated. The video RAM lives there until the VGA
> BAR is reprogrammed.
Right, but that's an MMIO area from guests PoV, even if in our
implementation is backed by RAM pages.
> The reason OVMF places itself there is because it is where the real SPI
> ROM is mapped in address space on a real system.
Just to clarify my understanding, this is not reported as a RAM region
to guests? So hvmloader or the domain builder populates this with RAM
to place OVMF, even if not reported as a RAM region in the memory map
(much like with ACPI tables for example).
I wonder whether we should have some kind of document or code comment
about the guest memory layout (maybe there's one and I'm missing it).
Thanks, Roger.
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