Re: [PATCH] x86/static-call: Remove early_boot_irqs_disabled check to fix Xen PVH dom0

[Peter Zijlstra]

On Sat, Dec 21, 2024 at 09:10:46PM +0000, Andrew Cooper wrote:
> __static_call_update_early() has a check for early_boot_irqs_disabled, but
> is used before early_boot_irqs_disabled is set up in start_kernel().
> 
> Xen PV has always special cased early_boot_irqs_disabled, but Xen PVH does
> not and falls over the BUG when booting as dom0.
> 
> It is very suspect that early_boot_irqs_disabled starts as 0, becomes 1 for
> a time, then becomes 0 again, but as this needs backporting to fix a
> breakage in a security fix, dropping the BUG_ON() is the far safer option.

Agreed.

> Fixes: 0ef8047b737d ("x86/static-call: provide a way to do very early static-call updates")
> Reported-by: Alex Zenla <alex@xxxxxxxxx>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Tested-by: Alex Zenla <alex@xxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
>  arch/x86/kernel/static_call.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> index 9eed0c144dad..9e51242ed125 100644
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -175,7 +175,6 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
>  noinstr void __static_call_update_early(void *tramp, void *func)
>  {
>  	BUG_ON(system_state != SYSTEM_BOOTING);
> -	BUG_ON(!early_boot_irqs_disabled);
>  	BUG_ON(static_call_initialized);
>  	__text_gen_insn(tramp, JMP32_INSN_OPCODE, tramp, func, JMP32_INSN_SIZE);
>  	sync_core();
> 
> base-commit: e84a3bf7f4aa669c05e3884497774148ac111468
> -- 
> 2.39.5
>