Re: [PATCH v2 1/8] xen/x86: Add initial x2APIC ID to the per-vLAPIC save area

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

On 23/05/2024 15:32, Roger Pau Monné wrote:
>>      case 0xb:
>> -        /*
>> -         * In principle, this leaf is Intel-only.  In practice, it is 
>> tightly
>> -         * coupled with x2apic, and we offer an x2apic-capable APIC 
>> emulation
>> -         * to guests on AMD hardware as well.
>> -         *
>> -         * TODO: Rework topology logic.
>> -         */
>> -        if ( p->basic.x2apic )
>> +        /* Don't expose topology information to PV guests */
> 
> Not sure whether we want to keep part of the comment about exposing
> x2APIC to guests even when x2APIC is not present in the host.  I think
> this code has changed and the comment is kind of stale now.

The comment is definitely stale. Nowadays x2APIC is fully supported by
AMD, as is leaf 0xb. The fact we emulate the x2APIC seems hardly
relevant in a CPUID leaf about topology. I could keep a note showing...

    /* Exposed alongside x2apic, as it's tightly coupled with it */

... although that's directly implied by the conditional.

>> +void vlapic_cpu_policy_changed(struct vcpu *v)
>> +{
>> +    struct vlapic *vlapic = vcpu_vlapic(v);
>> +    const struct cpu_policy *cp = v->domain->arch.cpu_policy;
>> +
>> +    /*
>> +     * Don't override the initial x2APIC ID if we have migrated it or
>> +     * if the domain doesn't have vLAPIC at all.
>> +     */
>> +    if ( !has_vlapic(v->domain) || vlapic->loaded.hw )
>> +        return;
>> +
>> +    vlapic->hw.x2apic_id = x86_x2apic_id_from_vcpu_id(cp, v->vcpu_id);
>> +    vlapic_set_reg(vlapic, APIC_ID, SET_xAPIC_ID(vlapic->hw.x2apic_id));
> 
> Nit: in case we decide to start APICs in x2APIC mode, might be good to
> take this into account here and use vlapic_x2apic_mode(vlapic) to
> select whether SET_xAPIC_ID() needs to be used or not:>
>     vlapic_set_reg(vlapic, APIC_ID,
>         vlapic_x2apic_mode(vlapic) ? vlapic->hw.x2apic_id
>                                  : SET_xAPIC_ID(vlapic->hw.x2apic_id));
> 
> Or similar.
> 

I like it. Sure.

>> +}
>> +
>>  int guest_wrmsr_apic_base(struct vcpu *v, uint64_t val)
>>  {
>>      const struct cpu_policy *cp = v->domain->arch.cpu_policy;
>> @@ -1449,7 +1465,7 @@ void vlapic_reset(struct vlapic *vlapic)
>>      if ( v->vcpu_id == 0 )
>>          vlapic->hw.apic_base_msr |= APIC_BASE_BSP;
>>  
>> -    vlapic_set_reg(vlapic, APIC_ID, (v->vcpu_id * 2) << 24);
>> +    vlapic_set_reg(vlapic, APIC_ID, SET_xAPIC_ID(vlapic->hw.x2apic_id));
>>      vlapic_do_init(vlapic);
>>  }
>>  
>> @@ -1514,6 +1530,16 @@ 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);
>>  
>> +    /*
>> +     * Loading record without hw.x2apic_id in the save stream, calculate 
>> using
>> +     * the traditional "vcpu_id * 2" relation. There's an implicit 
>> assumption
>> +     * that vCPU0 always has x2APIC0, which is true for the old relation, 
>> and
>> +     * still holds under the new x2APIC generation algorithm. While that 
>> case
>> +     * goes through the conditional it's benign because it still maps to 
>> zero.
>> +     */
>> +    if ( !vlapic->hw.x2apic_id )
>> +        vlapic->hw.x2apic_id = v->vcpu_id * 2;
>> +
>>      /* Skip fixups on xAPIC mode, or if the x2APIC LDR is already correct */
>>      if ( !vlapic_x2apic_mode(vlapic) ||
>>           (vlapic->loaded.ldr == good_ldr) )
>> diff --git a/xen/arch/x86/include/asm/hvm/hvm.h 
>> b/xen/arch/x86/include/asm/hvm/hvm.h
>> index 0c9e6f15645d..e1f0585d75a9 100644
>> --- a/xen/arch/x86/include/asm/hvm/hvm.h
>> +++ b/xen/arch/x86/include/asm/hvm/hvm.h
>> @@ -448,6 +448,7 @@ static inline void hvm_update_guest_efer(struct vcpu *v)
>>  static inline void hvm_cpuid_policy_changed(struct vcpu *v)
>>  {
>>      alternative_vcall(hvm_funcs.cpuid_policy_changed, v);
>> +    vlapic_cpu_policy_changed(v);
> 
> Note sure whether this call would better be placed in
> cpu_policy_updated() inside the is_hvm_vcpu() conditional branch.
> 
> hvm_cpuid_policy_changed() &c are just wrappers around the hvm_funcs
> hooks, pulling vlapic functions in there is likely to complicate the
> header dependencies in the long term.
> 

That's how it was in v1 and I moved it in v2 answering one of Jan's
feedback points.

I don't mind either way.

>>  }
>>  
>>  static inline void hvm_set_tsc_offset(struct vcpu *v, uint64_t offset,
>> diff --git a/xen/arch/x86/include/asm/hvm/vlapic.h 
>> b/xen/arch/x86/include/asm/hvm/vlapic.h
>> index 88ef94524339..e8d41313abd3 100644
>> --- a/xen/arch/x86/include/asm/hvm/vlapic.h
>> +++ b/xen/arch/x86/include/asm/hvm/vlapic.h
>> @@ -44,6 +44,7 @@
>>  #define vlapic_xapic_mode(vlapic)                               \
>>      (!vlapic_hw_disabled(vlapic) && \
>>       !((vlapic)->hw.apic_base_msr & APIC_BASE_EXTD))
>> +#define vlapic_x2apic_id(vlapic) ((vlapic)->hw.x2apic_id)
>>  
>>  /*
>>   * Generic APIC bitmap vector update & search routines.
>> @@ -107,6 +108,7 @@ int vlapic_ack_pending_irq(struct vcpu *v, int vector, 
>> bool force_ack);
>>  
>>  int  vlapic_init(struct vcpu *v);
>>  void vlapic_destroy(struct vcpu *v);
>> +void vlapic_cpu_policy_changed(struct vcpu *v);
>>  
>>  void vlapic_reset(struct vlapic *vlapic);
>>  
>> diff --git a/xen/include/public/arch-x86/hvm/save.h 
>> b/xen/include/public/arch-x86/hvm/save.h
>> index 7ecacadde165..1c2ec669ffc9 100644
>> --- 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 think Jan requested for this field to be checked to be 0 for
> incoming migrations, yet I don't see such check added.
> 

It's on the next patch, when the checks are moved into the check hook.
Doing it all as part of the same patch seemed dirty. I guess I can
invert them.

>>  };
>>  
>>  DECLARE_HVM_SAVE_TYPE(LAPIC, 5, struct hvm_hw_lapic);
>> diff --git a/xen/include/xen/lib/x86/cpu-policy.h 
>> b/xen/include/xen/lib/x86/cpu-policy.h
>> index d5e447e9dc06..392320b9adbe 100644
>> --- a/xen/include/xen/lib/x86/cpu-policy.h
>> +++ b/xen/include/xen/lib/x86/cpu-policy.h
>> @@ -542,6 +542,15 @@ int x86_cpu_policies_are_compatible(const struct 
>> cpu_policy *host,
>>                                      const struct cpu_policy *guest,
>>                                      struct cpu_policy_errors *err);
>>  
>> +/**
>> + * Calculates the x2APIC ID of a vCPU given a CPU policy
>> + *
>> + * @param p          CPU policy of the domain.
>> + * @param id         vCPU ID of the vCPU.
>> + * @returns x2APIC ID of the vCPU.
>> + */
>> +uint32_t x86_x2apic_id_from_vcpu_id(const struct cpu_policy *p, uint32_t 
>> id);
>> +
>>  #endif /* !XEN_LIB_X86_POLICIES_H */
>>  
>>  /*
>> diff --git a/xen/lib/x86/policy.c b/xen/lib/x86/policy.c
>> index f033d22785be..4cef658feeb8 100644
>> --- a/xen/lib/x86/policy.c
>> +++ b/xen/lib/x86/policy.c
>> @@ -2,6 +2,17 @@
>>  
>>  #include <xen/lib/x86/cpu-policy.h>
>>  
>> +uint32_t x86_x2apic_id_from_vcpu_id(const struct cpu_policy *p, uint32_t id)
>> +{
>> +    /*
>> +     * TODO: Derive x2APIC ID from the topology information inside `p`
>> +     *       rather than from vCPU ID. This bodge is a temporary measure
>> +     *       until all infra is in place to retrieve or derive the initial
>> +     *       x2APIC ID from migrated domains.
>> +     */
>> +    return vcpu_id * 2;
> 
> I don't think this builds?
> 
> Note the parameter is plain 'id' not 'vcpu_id'.
> 
> Thanks, Roger.

Bah. mid-patches after v1 don't get much attention. Ack.

Cheers,
Alejandro