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.coop...@citrix.com>
> ---
> CC: Jan Beulich <jbeul...@suse.com>
> CC: Roger Pau Monné <roger....@citrix.com>
>
> 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 <jbeul...@suse.com>
Jan
