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

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Tue, Sep 3, 2019 at 9:53 AM Jan Beulich <jbeulich@xxxxxxxx> 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?

Also, looks to me you should check whether the mfn is valid before
calling p2m->set_entry.

>
> > --- 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.

Perhaps instead of extending the HVMOP it would make more sense to
just add a xl config option that defines the "default" sve bit for
altp2m views in the domain?

Tamas

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