Re: [PATCH] revert "hvmloader: PA range 0xfc000000-0xffffffff should be UC"

[Andrew Cooper <amc96@srcf.net>]

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.

The reason OVMF places itself there is because it is where the real SPI
ROM is mapped in address space on a real system.

>> While this is benign to epte_get_entry_emt() as long as the IOMMU isn't
>> enabled for a guest, it becomes a very noticable problem otherwise: It
>> takes about half a minute for OVMF to decompress itself into its
>> designated address range.
>>
>> And even beyond OVMF there's no reason to have e.g. the ACPI memory
>> range marked UC.
>>
>> Fixes: c22bd567ce22 ("hvmloader: PA range 0xfc000000-0xffffffff should be UC")
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

~Andrew