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Re: [PATCH] tools/libs/light: set video_mem for PVH guests


  • To: Andrew Cooper <amc96@xxxxxxxx>
  • From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Date: Tue, 11 Jan 2022 12:26:24 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On Wed, Dec 08, 2021 at 02:00:48PM +0000, Andrew Cooper wrote:
> On 03/12/2021 07:30, Juergen Gross wrote:
> > The size of the video memory of PVH guests should be set to 0 in case
> > no value has been specified.
> >
> > Doing not so will leave it to be -1, resulting in an additional 1 kB
> > of RAM being advertised in the memory map (here the output of a PVH
> > Mini-OS boot with 16 MB of RAM assigned):
> >
> > Memory map:
> > 000000000000-0000010003ff: RAM
> > 0000feff8000-0000feffffff: Reserved
> > 0000fc008000-0000fc00803f: ACPI
> > 0000fc000000-0000fc000fff: ACPI
> > 0000fc001000-0000fc007fff: ACPI
> 
> The patch itself is fine, but some further observations based on the
> memory map alone.
> 
> It is rude to provide an unsorted memory map.
> 
> The LAPIC range is required to be reserved by the ACPI spec, missing
> here.  Conversely, it's unclear what the reserved region is trying to
> describe.

IIRC those are the special pages used by Xen console, store, identity
page tables, ioreq...

> Of the 3 ACPI ranges, one is RSDP (the first 64 bytes), one is the info
> block (4k), and one is the ACPI tables themselves.
> 
> RSDP really ought to be merged into the same block as the rest of the
> ACPI tables.
> 
> The info block must not be marked ACPI reclaimable RAM, because it is
> referenced by AML inside the DSDT/etc.  This is a very serious issue if
> the OS actually exercises its right to reclaim those regions and use
> them as RAM.

There's no DSDT on PVH, but I'm unsure whether it's also marked as
reclaimable RAM on HVM which does have a DSDT. Last two blocks should
likely be ACPI NVS I assume.

Thanks, Roger.



 


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