Re: [PATCH 1/8] x86/boot: Drop incorrect mapping at l2_xenmap[0]

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.11.2021 12:14, Andrew Cooper wrote:
> On 30/11/2021 10:33, Jan Beulich wrote:
>> On 30.11.2021 11:04, Andrew Cooper wrote:
>>> It has been 4 years since the default load address changed from 1M to 2M, 
>>> and
>>> _stext ceased residing in l2_xenmap[0].  We should not be inserting an 
>>> unused
>>> mapping.
>>>
>>> To ensure we don't create mappings accidentally, loop from 0 and obey
>>> _PAGE_PRESENT on all entries.
>>>
>>> Fixes: 7ed93f3a0dff ("x86: change default load address from 1 MiB to 2 MiB")
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> I guess this may be worth backporting despite not having any immediate
>> adverse effect.
> 
> I'd say so, yes.  I too can't see an adverse effect right now, but I'm
> definitely wary of stray executable mappings lying around.
> 
> 
> In principle, it would be nice to reclaim the 2M of space (which only
> exists for the MB1 path IIRC), but then we're getting into a position
> where xen_phys_start isn't really that any more.

Well, xen_phys_base might be slightly more accurate, but apart from that
I do think that we reclaim that space (as much as we did reclaim the 1Mb
before the change of the default load address):

    if ( efi_boot_mem_unused(&eb_start, &eb_end) )
    {
        reserve_e820_ram(&boot_e820, __pa(_stext), __pa(eb_start));
        reserve_e820_ram(&boot_e820, __pa(eb_end), __pa(__2M_rwdata_end));
    }
    else
        reserve_e820_ram(&boot_e820, __pa(_stext), __pa(__2M_rwdata_end));

Jan