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

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Andrew,

On 07/04/2016 06:43 PM, 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.
> 
> 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.
> 
> ~Andrew
> 

You're absolutely correct. However, on every VMENTRY Xen explicitly
flushes the TLBs of the currently active domain (and with it, of the
currently active (a)p2m table) and hence it should not result in an issue.

Thank you very much.

Best regards,
Sergej

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