Re: [PATCH 3/4] xen/arm: set vpidr on the pcpu where the vcpu will run

[Julien Grall <julien.grall@arm.com>]

On 16/02/2018 20:31, Stefano Stabellini wrote:
> On Fri, 16 Feb 2018, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 15/02/18 23:16, Stefano Stabellini wrote:
>>> On big.LITTLE systems not all cores have the same midr. Instead of
>>> initializing the vpidr to the boot cpu midr, set it to the value of the
>>> midr of the pcpu where the vcpu will run.
>>>
>>> This way, assuming that the vcpu has been created with the right pcpu
>>> affinity, the guest will be able to read the right vpidr value, matching
>>> the one of the physical cpu.
>>>
>>> Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
>>> ---
>>>    xen/arch/arm/domain.c | 19 ++++++++++++++++---
>>>    1 file changed, 16 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>>> index 532e824..280125f 100644
>>> --- a/xen/arch/arm/domain.c
>>> +++ b/xen/arch/arm/domain.c
>>> @@ -315,6 +315,22 @@ static void schedule_tail(struct vcpu *prev)
>>>    static void continue_new_vcpu(struct vcpu *prev)
>>>    {
>>>        current->arch.actlr = READ_SYSREG32(ACTLR_EL1);
>>> +    /*
>>> +     * Default the virtual ID to match the physical.
>>> +     *
>>> +     * In case the big.LITTLE systems, a guest should be created with
>>> +     * cpu affinity set so that all vcpus run on the same kind of pcpus.
>>> +     * Warn if it is not the case.
>>
>> continue_new_vcpu is only called once at domain creation. So this looks
>> pointless to check that here and probably in ctxt_switch_to.
>>
>> But I don't want to see such check at every context switch. This is expensive
>> and we should not impact all the platforms for the benefits of an unsafe
>> configuration.
>>
>> If you really want to do that, then it should only be done when the vCPU is
>> migrating. That will reduce a lot the performance impact.
> 
> I don't want a check for every context switch either. I added it here
> because continue_new_vcpu is only called once per vcpu at domain
> creation -- it is a one time check. vcpus are supposed to be pinned (or
> cpu affinity specified) anyway, so I thought I wouldn't add the check in
> vcpu_migrate too. In any case, I am also happy to remove the check
> completely, as we have already warned the user enough.
If you agree that continue_new_vcpu is only called once per vCPU. Then I 
am not sure to understand the purpose of the check. What are you trying 
to warn the user with that?

Cheers,

-- 
Julien Grall

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