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

[Julien Grall <julien@xxxxxxx>]

Hi,

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?

> A 12 TiB host was certified successfully for use with Xen 4.14 as per
> https://www.suse.com/nbswebapp/yesBulletin.jsp?bulletinNumber=150753.
> This in particular included running as many guests (2 TiB each) as
> possible in parallel, to actually prove that all the memory can be used
> like this. It may be relevant to note that the Optane memory there was
> used in memory-only mode, with DRAM acting as cache.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- 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

>   
>   Hosts with more memory are supported, but not security supported.
>   
> @@ -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.

> +
> +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.
Cheers,

[1] https://wiki.xenproject.org/wiki/Xen_Project_Release_Features

--
Julien Grall