Re: [Xen-devel] [PATCH v1] x86/altp2m: Add hypercall to create a new view and set sve bits

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 04.09.2019 15:14, Jan Beulich wrote:
> On 04.09.2019 13:51, Alexandru Stefan ISAILA wrote:
>>
>>
>> On 03.09.2019 18:52, Jan Beulich wrote:
>>> On 02.09.2019 10:11, Alexandru Stefan ISAILA wrote:
>>>> @@ -1355,6 +1355,23 @@ void p2m_init_altp2m_ept(struct domain *d, unsigned 
>>>> int i)
>>>>        ept = &p2m->ept;
>>>>        ept->mfn = pagetable_get_pfn(p2m_get_pagetable(p2m));
>>>>        d->arch.altp2m_eptp[i] = ept->eptp;
>>>> +
>>>> +    if ( set_sve )
>>>> +    {
>>>> +        unsigned long gfn = 0, max_gpfn = domain_get_maximum_gpfn(d);
>>>> +
>>>> +        for( ; gfn < max_gpfn; ++gfn )
>>>> +        {
>>>> +            mfn_t mfn;
>>>> +            p2m_access_t a;
>>>> +            p2m_type_t t;
>>>> +
>>>> +            altp2m_get_effective_entry(p2m, _gfn(gfn), &mfn, &t, &a,
>>>> +                                       AP2MGET_query);
>>>> +            p2m->set_entry(p2m, _gfn(gfn), mfn, PAGE_ORDER_4K, t, a, 
>>>> true);
>>>> +
>>>> +        }
>>>> +    }
>>>>    }
>>>
>>> How long is this loop going to take for a huge guest? IOW how
>>> come there's no preemption in here, or some other mechanism
>>> to bound execution time?
>>
>> Because this is done for the initialization of a new view and the p2m is
>> locked.
> 
> Well, this makes handling this the way you want it close to
> impossible, but is not an argument against the need for preemption
> here. Just like it had turned out to be unreasonable to
> preemptively handle other P2M adjustments (which is why
> p2m-ept.c:resolve_misconfig() and p2m-pt.c:do_recalc() got
> introduced), I'm afraid you'll have to use some other technique
> here (possibly building on top of the mentioned functions).
> 

I think that the mechanism from p2m_set_mem_access_multi() can suit this 
case, start the loop, set ,if(hypercall_preempt_check()) rc = 
next_unset_gfn;

And for this to work it should have a new "start_gfn" parameter so the 
caller can issue multiple hypercalls until gfn == max_gfn.

Alex
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