On 28.08.2025 17:03, Andrew Cooper wrote:
> The FRED on-stack format is larger than the IDT format, but is by and large
> compatible. FRED reuses space above cs and ss for extra metadata, some of
> which is purely informational, and some of which causes additional effects in
> ERET{U,S}.
>
> Follow Linux's choice of naming for fred_{c,s}s structures, to make it very
> clear at the point of use that it's dependent on FRED.
>
> There is also the event data field and reserved fields, but we cannot include
> these in struct cpu_user_regs without reintroducing OoB structure accesses in
> the non-FRED case. See commit 6065a05adf15 ("x86/traps: 'Fix' safety of
> read_registers() in #DF path"). for more details.
>
> Instead, use a new struct fred_info and position it suitably in struct
> cpu_info. This boundary will be loaded into MSR_FRED_RSP_SL0, and must be
> 64-byte aligned.
>
> This does add 16 bytes back into struct cpu_info, undoing the saving we made
> by dropping the vm86 data segment selectors.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <[email protected]>
Reviewed-by: Jan Beulich <[email protected]>
