Re: [PATCH 08/15] x86/PVH: split pvh_setup_acpi()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.10.2026 11:31, Roger Pau Monné wrote:
> On Tue, Oct 13, 2026 at 06:04:37PM +0200, Jan Beulich wrote:
>> On 13.10.2026 17:52, Roger Pau Monné wrote:
>>> On Mon, Oct 12, 2026 at 10:23:16AM +0200, Jan Beulich wrote:
>>>> All E820 manipulations want to happen before the initrd is placed, or else
>>>> pvh_steal_ram() may steal from the tail part of it. As a second step,
>>>> split placing ACPI data from actually populating it.
>>>
>>> Would it be less intrusive to duplicate the memory map (like it's done
>>> in a change ahead of this one), and subtract the kernel and initrd
>>> region from that memory map, then use it as the basis for
>>> pvh_steal_ram()?
>>>
>>> We could adjust/replace pvh_add_mem_range() so it allows changing
>>> regions from RAM to other types.  That would seem to be less churn
>>> overall.  Maybe you did already consider this and found further
>>> problem with this approach that I'm failing to see?
>>
>> No, I didn't try that route. In fact despite the order now this patch
>> (and maybe also the VMX one) pre-dates the E820-copying one. The one
>> aspect I consider problematic with the suggested alternative is that
>> then two E820 maps need updating: The one with kernel (not sure about
>> initrd; see below) subtracted out, and the one we actually mean to hand
>> to Dom0.
>>
>> As to overall churn: I'm not sure which one would cause less. Certainly
>> the patch here would remain largely unaltered, as the splitting is
>> going to be necessary no matter what.
> 
> I'm possibly missing something, but the splitting won't be strictly
> necessary?
> 
> We could do:
> 
>  * Populate p2m
>  * Place kernel
>  * Place ACPI (also VMX metadata)
>  * Place initrd

The goal is to directly map the initrd. That direct-mapping (or at least
the determination of where it is to go) needs to happen before populating
the (rest of) the P2M (as otherwise this would populate the respective
part of the P2M). Else we'll continue to see the problem that Marek
reported. That is:

 * Place kernel
 * Place ACPI (maybe VMX metadata)
 * Place initrd (possibly direct-mapping it)
 * Populate P2M
 * Load kernel and initrd
 * Fill ACPI (maybe VMX metadata)

(Of course the order of the latter two items doesn't matter much.)

Even if we didn't direct-map the initrd along with placing it, we'd
still need to tell populate_p2m() to skip that region. I.e. the split
between placing and loading/filling would remain.

Jan