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

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

[Sean Christopherson]

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>
---
 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
-- 
2.52.0.322.g1dd061c0dc-goog

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

[Igor Mammedov]

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