Re: [PATCH] x86: allow non-BIGMEM configs to boot on >= 16Tb systems

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.12.2023 15:54, Roger Pau Monné wrote:
> On Wed, Jun 07, 2023 at 08:17:30AM +0200, Jan Beulich wrote:
>> While frame table setup, directmap init, and boot allocator population
>> respect all intended bounds, the logic passing memory to the heap
>> allocator which wasn't passed to the boot allocator fails to respect
>> max_{pdx,pfn}. This then typically triggers the BUG() in
>> free_heap_pages() after checking page state, because of hitting a struct
>> page_info instance which was set to all ~0.
>>
>> Of course all the memory above the 16Tb boundary is still going to
>> remain unused; using it requires BIGMEM=y. And of course this fix
>> similarly ought to help BIGMEM=y configurations on >= 123Tb systems
>> (where all the memory beyond that boundary continues to be unused).
>>
>> Fixes: bac2000063ba ("x86-64: reduce range spanned by 1:1 mapping and frame 
>> table indexes")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -1722,15 +1722,16 @@ void __init noreturn __start_xen(unsigne
>>  
>>      if ( max_page - 1 > virt_to_mfn(HYPERVISOR_VIRT_END - 1) )
>>      {
>> -        unsigned long limit = virt_to_mfn(HYPERVISOR_VIRT_END - 1);
>> +        unsigned long lo = virt_to_mfn(HYPERVISOR_VIRT_END - 1);
>> +        unsigned long hi = pdx_to_pfn(max_pdx - 1) + 1;
> 
> Maybe use max_page to avoid the pdx_to_pfn() call?  (And is also more
> in context with the condition on the outside if).

You mean

        unsigned long hi = min(pdx_to_pfn(max_pdx - 1) + 1, max_page);

? I could switch to that, yes. I wouldn't feel well switching to using
just max_page, especially with me having nowhere to (reasonably) test.

Jan