Re: [PATCH] SUPPORT.md: extend security support for hosts to 12 TiB of memory

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.04.2022 17:15, Julien Grall wrote:
> On 06/04/2022 15:44, Jan Beulich wrote:
>> c49ee0329ff3 ("SUPPORT.md: limit security support for hosts with very
>> much memory"), as a result of XSA-385, restricted security support to
>> 8 TiB of host memory. Extend this to 12 TiB, putting in place a guest
>> restriction to 8 TiB in exchange.
> 
> And this is even without CONFIG_BIGMEM?

Yes. BIGMEM only matters when memory extends past the 16 TiB boundary
(i.e. when frame numbers with ore than 32 significant bits appear).

>> --- a/SUPPORT.md
>> +++ b/SUPPORT.md
>> @@ -50,7 +50,7 @@ For the Cortex A57 r0p0 - r1p1, see Erra
>>   
>>   ### Physical Memory
>>   
>> -    Status: Supported up to 8 TiB
>> +    Status: Supported up to 12 TiB
> 
> I am afraid this limit is going to be too high for Arm. Even the 
> previous one was technically incorrect. From [1], it should be:
>    - 5TB for arm64
>    - 16GB for arm32

May I ask that you submit a patch correcting this, and I'll rebase
on top of that? I can't really fit such an adjustment under the
umbrella of the title and purpose of this change.

>> @@ -121,6 +121,14 @@ ARM only has one guest type at the momen
>>   
>>       Status: Supported
>>   
>> +## Guest Limits
>> +
>> +### Memory
>> +
>> +    Status: Supported up to 8 TiB
> 
> For Arm, this should be limited to 1TB for arm64 and 16GB for arm32.

Sure, will do.

>> +
>> +Guests with more memory are supported, but not security supported.
> 
> d->max_pages is a 32-bit value. So Xen can effectively only support up 
> to 16TB of memory. AFAICT, it would require quite a bit rework to lift 
> that limit. So I think it would be better to spell out the upper limit.

Same here.

Jan