Re: [PATCH v2 19/23] x86/pv: Adjust GS handling for FRED mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2025 17:04, Andrew Cooper wrote:
> When FRED is active, hardware automatically swaps GS when changing privilege,
> and the SWAPGS instruction is disallowed.
> 
> For native OSes using GS as the thread local pointer this is a massive
> improvement on the pre-FRED architecture, but under Xen it makes handling PV
> guests more complicated.  Specifically, it means that GS_BASE and GS_SHADOW
> are the opposite way around in FRED mode, as opposed to IDT mode.
> 
> This leads to the following changes:
> 
>   * In load_segments(), we have to load both GSes.  Account for this in the
>     SWAP() condition and avoid the path with SWAGS.
> 
>   * In save_segments(), we need to read GS_KERN rather than GS_BASE.

GS_SHADOW in our terminology, that is. (Also again in code comments,
and there's also a variable named gs_kern.)

>   * In toggle_guest_mode(), we need to emulate SWAPGS.
> 
>   * In do_set_segment_base(), merge the SEGBASE_GS_{USER,KERNEL} cases and
>     take FRED into account when choosing which base to update.
> 
>     SEGBASE_GS_USER_SEL was already an LKGS invocation (decades before FRED)
>     so under FRED needs to be a simple MOV %gs.  Simply skip the SWAPGSes.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> v2:
>  * New
> 
> I think this functions, but it's not ideal.  The conditions are asymmetric and
> awkward.

It's not as bad as I expect it to be after reading this remark.

Preferably with the naming adjusted:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan