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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

> --- 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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