Re: [PATCH 1/4] x86/PVH: de-duplicate mappings for first Mb of Dom0 memory

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.08.2021 15:27, Andrew Cooper wrote:
> On 31/08/2021 14:19, Jan Beulich wrote:
>>>>> @@ -1095,6 +1101,17 @@ static int __init pvh_setup_acpi(struct
>>>>>          nr_pages = PFN_UP((d->arch.e820[i].addr & ~PAGE_MASK) +
>>>>>                            d->arch.e820[i].size);
>>>>>  
>>>>> +        /* Memory below 1MB has been dealt with by pvh_populate_p2m(). */
>>>>> +        if ( pfn < PFN_DOWN(MB(1)) )
>>>>> +        {
>>>>> +            if ( pfn + nr_pages <= PFN_DOWN(MB(1)) )
>>>>> +                continue;
>>>>> +
>>>>> +            /* This shouldn't happen, but is easy to deal with. */
>>>> I'm not sure this comment is helpful.
>>>>
>>>> Under PVH, there is nothing special about the 1M boundary, and we can
>>>> reasonably have something else here or crossing the boundary.
>>> As long as we have this special treatment of the low Mb, the boundary
>>> is meaningful imo. I'd see the comment go away when the handling in
>>> general gets streamlined.
>> I should have added: And as long as Dom0's E820 map gets cloned from
>> the host's, which will necessarily consider the 1Mb boundary special.
> 
> Not when you're booting virtualised in the first place.

You mean when Xen itself runs in PVH (not HVM) mode, and then in turn
has a PVH Dom0? And if the underlying Xen has not in turn cloned
the E820 from the host's? That's surely too exotic a case to warrant
removing this comment. If you insist, I can mention that case as a
possible exception.

Jan