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

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

On 07/04/2016 07:44 PM, Julien Grall wrote:
> 
> 
> On 04/07/16 17:56, Sergej Proskurin wrote:
>> 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.
> 
> VMENTRY is x86 not ARM. So are you sure you looked at the correct code?
> 
> Regards,
> 

This is true. I just use the term VMENTER for describing transitions to
guests on both, x86 and ARM. In ./xen/arch/arm/domain.c the function
ctxt_switch_to calls p2m_restore_state on every context switch, wich in
turn loads the VTTBR associated to the domain and flushes the TLBs.

Best regards,
Sergej

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