Re: [PATCH] x86/efi: Skip FPU save/restore for idle vCPU in EFI, runtime path

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 6/12/26 5:41 PM, Bernhard Kaindl wrote:
> Hi Anthony, could you test this patch which exactly applies the changes 
> Jan suggested? Summary:
> Guard both EFI runtime FPU calls with !is_idle_vcpu() to skip save/restore
> for idle vCPUs, which don't have an FPU context to save/restore,
> much like the calls are guarded in __context_switch(),
> where save/restore is done only for non-idle vCPUs.
> As these simple guards should preferably go into Xen 4.22: Please test 
> if there are any further regressions with the 'cmos-rtc-probe' 
> workaround you just added removed to check if guarding the assertions as 
> Jan suggested is enough to fix the issues triggered on your machine. 
> Thanks, Bernhard The patch to test follows: [PATCH] x86/efi: Skip FPU 
> save/restore for idle vCPU in EFI, runtime path
> Anthony reported a boot-time crash in init_xen_time() via efi_get_time()
> on a Broadwell-D system:
>    Assertion '!is_idle_vcpu(v)' failed at arch/x86/i387.c:195
> The failing path is an EFI runtime call reached early during boot,
> where current may still be the idle vCPU.
> This became fragile after the lazy-FPU removal cleanup series.
> In 1792bb9a99d2 ("x86: Cleanup cr0.TS flag handling"),
> efi_rs_enter() was changed from save_fpu_enable() to vcpu_save_fpu(curr),
> which unconditionally asserts !is_idle_vcpu(v)
> so an EFI runtime call in idle context now asserts.
> Likewise, in dba44e051209 ("x86: Remove fully_eager_fpu"),
> efi_rs_leave() was changed to call vcpu_restore_fpu(curr),
> which has the same assertion and can fail for the same reason.
> Guard both EFI runtime FPU calls with !is_idle_vcpu() to skip save/restore
> for idle vCPUs, which don't have an FPU context to save/restore,
> much like the calls are guarded in __context_switch(),
> where save/restore is done only for non-idle vCPUs.
> Fixes: 1792bb9a99d2 ("x86: Cleanup cr0.TS flag handling")
> Fixes: dba44e051209 ("x86: Remove fully_eager_fpu")
> Reported-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>
> ---

Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Thanks.

~ Oleksii