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

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

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.

Alex

> 
>> --- a/xen/include/public/hvm/hvm_op.h
>> +++ b/xen/include/public/hvm/hvm_op.h
>> @@ -244,6 +244,7 @@ struct xen_hvm_altp2m_view {
>>       /* Create view only: default access type
>>        * NOTE: currently ignored */
>>       uint16_t hvmmem_default_access; /* xenmem_access_t */
>> +    uint8_t set_sve; /* bool value */
>>   };
> 
> This interface is, given the right configuration, available to
> guests. Hence you can't simply add a field here. Just consider
> what happens for an existing caller when there is random data
> in the field you now assign a meaning.
> 
> Furthermore, according to common practice elsewhere, the new
> trailing padding field should be made explicit, and checked to
> hold zero on input.
> 
> Jan

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