Re: [PATCH v1 22/28] x86/hvm: avic: ensure SVM AVIC supports xAPIC operation only

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/11/2026 1:33 am, dmukhin@xxxxxxxx wrote:
> From: Denis Mukhin <dmukhin@xxxxxxxx>
>
> xAVIC does not support x2APIC operation (yet).
>
> Until x2APIC mode is supported, don't advertise x2APIC to HVM
> guests when SVM AVIC is enabled.
>
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
> ---
>  xen/arch/x86/cpu-policy.c          |  3 ++-
>  xen/arch/x86/hvm/svm/svm.c         | 12 ++++++++++++
>  xen/arch/x86/hvm/vmx/vmx.c         |  4 ++++
>  xen/arch/x86/include/asm/hvm/hvm.h | 15 ++++++++++++++-
>  4 files changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
> index 48a3185eed8d..ca9b1a523570 100644
> --- a/xen/arch/x86/cpu-policy.c
> +++ b/xen/arch/x86/cpu-policy.c
> @@ -764,7 +764,8 @@ static void __init calculate_hvm_max_policy(void)
>       * (x2)APIC isn't enabled.
>       */
>      __set_bit(X86_FEATURE_APIC, fs);
> -    __set_bit(X86_FEATURE_X2APIC, fs);
> +    if ( hvm_x2apic_supported() )
> +        __set_bit(X86_FEATURE_X2APIC, fs);

The reasoning is backwards, and breaks things.

HVM guests do support x2APIC, and have seen the feature bit for a decade
or more.

Clobbering this bit will cause the Xapi toolstack to block migration. 
The xl toolstack doesn't do the relevant safety checks (yet) and the VM
will explode as a result if it was actually using x2APIC.

x2AVIC is an acceleration.  It's fine for the acceleration not to be
turned on, but it is not fine to hide x2APIC as a consequence.

Both the Viridian spec and Xen itself have hints in the virtualised
leaves to tell a guest whether it ought to prefer using xAPIC or x2APIC
mode.  Under plain virt, x2APIC is vastly preferable because the
RD/WRMSR exit gives you all the information you require, whereas xAPIC
requires instruction emulation to figure out the register was being
accessed.

~Andrew