Re: [PATCH 1/6] xen/x86: Add initial x2APIC ID to the per-vLAPIC save area

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.01.2024 16:38, Alejandro Vallejo wrote:
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -288,7 +288,10 @@ void update_guest_memory_policy(struct vcpu *v,
>  static void cpu_policy_updated(struct vcpu *v)
>  {
>      if ( is_hvm_vcpu(v) )
> +    {
>          hvm_cpuid_policy_changed(v);
> +        vlapic_cpu_policy_changed(v);
> +    }
>  }

This is a layering violation imo; hvm_cpuid_policy_changed() wants
to call vlapic_cpu_policy_changed().

> @@ -1083,6 +1083,22 @@ static void set_x2apic_id(struct vlapic *vlapic)
>      vlapic_set_reg(vlapic, APIC_LDR, apic_ldr);
>  }
>  
> +void vlapic_cpu_policy_changed(struct vcpu *v)
> +{
> +    struct vlapic *vlapic = vcpu_vlapic(v);
> +    struct cpu_policy *cp = v->domain->arch.cpu_policy;

const please

> @@ -1514,6 +1530,13 @@ static void lapic_load_fixup(struct vlapic *vlapic)
>      const struct vcpu *v = vlapic_vcpu(vlapic);
>      uint32_t good_ldr = x2apic_ldr_from_id(vlapic->loaded.id);
>  
> +    /*
> +     * Guest with hardcoded assumptions about x2apic_id <-> vcpu_id
> +     * mappings. Recreate the mapping it used to have in old host.
> +     */
> +    if ( !vlapic->hw.x2apic_id )
> +        vlapic->hw.x2apic_id = v->vcpu_id * 2;

This looks to depend upon it only ever being vCPU which may get a (new
style) APIC ID of 0. I think such at least wants mentioning in the
comment.

> --- a/xen/include/public/arch-x86/hvm/save.h
> +++ b/xen/include/public/arch-x86/hvm/save.h
> @@ -394,6 +394,8 @@ struct hvm_hw_lapic {
>      uint32_t             disabled; /* VLAPIC_xx_DISABLED */
>      uint32_t             timer_divisor;
>      uint64_t             tdt_msr;
> +    uint32_t             x2apic_id;
> +    uint32_t             rsvd_zero;
>  };

I can't spot any checking of this last field indeed being zero.

Jan