Re: [Xen-devel] [PATCH] x86/mm/pat: Fix BUG_ON in mmap_mem on QEMU/i386

[David Vrabel]

On 08/04/16 17:34, Toshi Kani wrote:
> On Tue, 2016-04-05 at 09:24 -0600, Toshi Kani wrote:
>> +xen-devl
>>
>> On Tue, 2016-04-05 at 13:09 +0200, Borislav Petkov wrote:
>>> On Fri, Apr 01, 2016 at 04:19:45PM -0600, Toshi Kani wrote:
>>>>
>  :
>>>>
>>>> When the system does not have much memory, 'high_memory' points to
>>>
>>> What does "much memory" mean, exactly?
>>
>> I meant to say when a 32-bit system does not have ZONE_HIGHMEM,
>> __pa(high_memory) points to the maximum memory address + 1.
>>
>> I will remove this sentence since it is irrelevant to this BUG_ON.  Even
>> if a 32-bit system does have ZONE_HIGHMEM, slow_virt_to_phys() still
>> returns 0 for high_memory because it is set to the maximum direct mapped
>> address + 1 in this case.  This address is not covered by page table,
>> either.
>>
>> But this made me realized that this high_memory check can be harmful in
>> such case, ie. __pa(high_memory) is not the maximum memory address when
>> ZONE_HIGHMEM is present.
>>
>> I assume when this code block was originally added, legacy systems
>> without MTRRs did not have ZONE_HIGHMEM.  However, MTRRs are also
>> disabled on Xen. Reactivating this code may cause an issue on Xen 32-bit
>> guests with ZONE_HIGHMEM.
>>
>> Question to Xen folks: Does Xen support 32-bit guests with ZONE_HIGHMEM?
>>
>> If yes, a safer fix may be to remove this code block since it was
>> deadcode anyway...
> 
> I have not heard a confirmation from Xen folks, but I believe ZONE_HIGHMEM
> is supported on 32-bit Xen guests.  So, unless someone has an objection, I
> am going to remove this code block in the next version of this patch.

32-bit Xen guests have highmem, yes.

David