Re: [kvm-unit-tests PATCH] x86/cstart: Delete "local" version of save_id() to fix x2APIC SMP bug

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 18 Dec 2025 15:26:18 -0800
Sean Christopherson <seanjc@google.com> wrote:

> Delete cstart.S's version of save_id() to play nice with 32-bit SMP tests
> on CPUs that support x2APIC.  The local version assumes xAPIC mode and so
> depending on the underlying "hardware" (e.g. AVIC vs. APICv vs. emulated,
> and with or without KVM_X86_QUIRK_LAPIC_MMIO_HOLE), may mark the wrong CPU
> as being online.
> 
> Fixes: 0991c0ea ("x86: fix APs with APIC ID more that 255 not showing in id_map")
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Acked-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  x86/cstart.S | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/x86/cstart.S b/x86/cstart.S
> index dafb330d..49ba4818 100644
> --- a/x86/cstart.S
> +++ b/x86/cstart.S
> @@ -84,13 +84,6 @@ prepare_32:
>  
>  smp_stacktop:	.long stacktop - per_cpu_size
>  
> -save_id:
> -	movl $(APIC_DEFAULT_PHYS_BASE + APIC_ID), %eax
> -	movl (%eax), %eax
> -	shrl $24, %eax
> -	lock btsl %eax, online_cpus
> -	retl
> -
>  ap_start32:
>  	setup_segments
>  	mov $-per_cpu_size, %esp
> 
> base-commit: 31d91f5c9b7546471b729491664b05c933d64a7a