Re: [Xen-devel] [PATCH] xen/arm: Implement MPIDR per VCPU

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 06/12/2013 04:11 PM, Ian Campbell wrote:

> On Fri, 2013-06-07 at 12:38 +0100, Julien Grall wrote:
>> Use different affinity for each VCPU and always expose an SMP systems to
>> the guest.
>>
>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
>> ---
>>  xen/arch/arm/domain.c           |   11 +++++++++--
>>  xen/include/asm-arm/processor.h |    6 ++++++
>>  2 files changed, 15 insertions(+), 2 deletions(-)
>>
>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>> index ff1410d..4654c9b 100644
>> --- a/xen/arch/arm/domain.c
>> +++ b/xen/arch/arm/domain.c
>> @@ -150,7 +150,8 @@ static void ctxt_switch_to(struct vcpu *n)
>>      isb();
>>
>>      WRITE_SYSREG32(n->domain->arch.vpidr, VPIDR_EL2);
>> -    WRITE_SYSREG(n->domain->arch.vmpidr, VMPIDR_EL2);
>> +    WRITE_SYSREG(n->domain->arch.vmpidr | (n->vcpu_id << MPIDR_AFF0_SHIFT),
>> +                 VMPIDR_EL2);
>
> Perhaps we should add v->arch.vmpidr and use that instead?

As it's a read-only register, why can't we recreate it at each context
switch? Adding a new field per cpu is a waste of space mainly when the
vcpu structure must not be greater than a page.

>>      /* VGIC */
>>      gic_restore_state(n);
>> @@ -495,7 +496,13 @@ int arch_domain_create(struct domain *d, unsigned int 
>> domcr_flags)
>>
>>      /* Default the virtual ID to match the physical */
>>      d->arch.vpidr = boot_cpu_data.midr.bits;
>> -    d->arch.vmpidr = boot_cpu_data.mpidr.bits;
>> +    /*
>> +     * Expose an SMP systems and remove the AFF0. It will be replace by
>> +     * the VPCU ID
>
> I wonder if that instead of basing this on the underlying processor we
> should fabricate an entirely virtual one?

Hum .. right, AFF1 could be different to 0.
Is it okay if xen exposes all the vcpus in the same cluster?

FYI, Linux uses the following approach to interpret MPIDR,
in case of an SMP support without mutli-thread support):
  - AFF0: Processors
  - AFF1: Clusters
  - AFF2: Unused

On the TC2 with big.LITTLE enabled, A7 and A5 are on different sockets,
ie AFF1 fields are not the same. But it's only used for PMU (Performance
monitor unit).

-- 
Julien

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