Re: [PATCH for-4.22] x86/fred: Enable FRED by default on AMD systems

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.06.2026 11:36, Andrew Cooper wrote:
> On 03/06/2026 7:27 am, Jan Beulich wrote:
>> On 02.06.2026 18:45, Andrew Cooper wrote:
>>> FRED is now believed to be complete for AMD systems, and has had its tyres
>>> kicked by both XenServer and AMD.  Enable FRED by default on capable AMD
>>> systems (Zen6 and later).
>>>
>>> Support on Intel is still not yet complete.  Leave it as tech preview and 
>>> not
>>> security supported.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
>> with one remark:
>>
>>> --- a/docs/misc/xen-command-line.pandoc
>>> +++ b/docs/misc/xen-command-line.pandoc
>>> @@ -1259,12 +1259,12 @@ does not provide `VM_ENTRY_LOAD_GUEST_PAT`.
>>>  ### fred (x86)
>>>  > `= <bool>`
>>>  
>>> -> Default: `false`
>>> +> Default: `true` on AMD, `false` otherwise
>>>  
>>>  Flexible Return and Event Delivery is an overhaul of interrupt, exception 
>>> and
>>>  system call handling, fixing many corner cases in the x86 architecture, and
>>> -expected in hardware from 2025.  Support in Xen is a work in progress and
>>> -disabled by default.
>>> +expected in hardware from 2026.  FRED is fully supported on AMD hardware.
> 
> Along with the year, I suppose I should name the CPUs specifically now
> that they're public.
> 
>>> +Intel hardware is still tech preview and not security supported.
>> Is having "security" here really appropriate? There's a functional issue to 
>> be
>> sorted. That's expressed by "tech preview", yes, but it still feels somewhat
>> odd. If you want to keep the word, may I suggest to insert "in particular"?
> 
> I was just trying to make things clearer.  One of the common audience
> groups for this file is not liable to know Xen's precise definitions of
> experimental/tech-preview/supported.
> 
>> May I further suggest "On Intel hardware it is ..."?
> 
> How does this look?
> 
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index ef3c7371895b..ff6d08affb63 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1259,12 +1259,13 @@ does not provide `VM_ENTRY_LOAD_GUEST_PAT`.
>  ### fred (x86)
>  > `= <bool>`
>  
> -> Default: `false`
> +> Default: `true` on AMD, `false` otherwise
>  
>  Flexible Return and Event Delivery is an overhaul of interrupt, exception and
> -system call handling, fixing many corner cases in the x86 architecture, and
> -expected in hardware from 2025.  Support in Xen is a work in progress and
> -disabled by default.
> +system call handling, fixing many corner cases in the x86 architecture, and 
> is
> +available on Intel Panther Lake and Diamond Rapids CPUs, and AMD Zen6 CPUs.
> +FRED is fully supported on AMD hardware.  On Intel hardware it is still tech
> +preview, and in particular not security supported.
>  
>  ### gnttab
>  > `= List of [ max-ver:<integer>, transitive=<bool>, transfer=<bool> ]`
> 
> 
> ?

Looks good, thanks.

Jan