Re: [Xen-devel] [PATCH 04/18] arm/altp2m: Add altp2m init/teardown routines.

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

On 07/04/2016 08:18 PM, Julien Grall wrote:
> 
> 
> On 04/07/16 17:43, Andrew Cooper wrote:
>> On 04/07/16 17:40, Sergej Proskurin wrote:
>>>
>>>>>    {
>>>>> -    struct p2m_domain *p2m = &d->arch.p2m;
>>>>> +    int ret = 0;
>>>>> +
>>>>> +    spin_lock_init(&p2m->lock);
>>>>> +    INIT_PAGE_LIST_HEAD(&p2m->pages);
>>>>> +
>>>>> +    spin_lock(&p2m->lock);
>>>>> +
>>>>> +    p2m->domain = d;
>>>>> +    p2m->access_required = false;
>>>>> +    p2m->mem_access_enabled = false;
>>>>> +    p2m->default_access = p2m_access_rwx;
>>>>> +    p2m->p2m_class = p2m_host;
>>>>> +    p2m->root = NULL;
>>>>> +
>>>>> +    /* Adopt VMID of the associated domain */
>>>>> +    p2m->vmid = d->arch.p2m.vmid;
>>>> It looks like to me that re-using the same VMID will require more TLB
>>>> flush (such as when a VCPU is migrated to another physical CPU). So
>>>> could you explain why you decided to re-use the same VMID?
>>>>
>>> Please correct me if I am wrong, but I associate a VMID with an entire
>>> domain. Since, the altp2m view still belongs to the same domain
>>> (p2m_init_one is called only from p2m_init_altp2m), the code re-uses the
>>> old VMID.
>>
>> (I am not an ARM expert but) looking into VMIDs from the last time, they
>> are the TLB tag for the address space in use.
> 
> Correct.
> 
>>
>> Does ARM have shared TLBs between multiple cores?  If so, you must a
>> separate VMID, otherwise an ALT2PM used by one vcpu could cause a
>> separate vcpu with a different ALTP2M to reuse the wrong translation.
> 
> From the ARM ARM, I cannot rule out that TLBs cannot be shared from
> multiple cores (CC a couple of ARM folks to confirm).
> 
> Nevertheless, using a different VMID per P2M would avoid to care about
> flushing when moving the vCPU around.
> 

I see what you mean. This would definitely improve the context switch
performance. That is, instead of adopting the hostp2m's vmid during
altp2m table initialization, I will allocate a new VMID for every altp2m
table --as you said before. This would simply eliminate the need for
explicit TLB flushing. Thank you very much.

> Regards,
> 

Best regards,
Sergej

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