Re: [Xen-devel] [PATCH v10 6/6] x86/ioreq server: Synchronously reset outstanding p2m_ioreq_server entries when an ioreq server unmaps.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.04.17 at 14:24, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> After an ioreq server has unmapped, the remaining p2m_ioreq_server
> entries need to be reset back to p2m_ram_rw. This patch does this
> synchronously by iterating the p2m table.
> 
> The synchronous resetting is necessary because we need to guarantee
> the p2m table is clean before another ioreq server is mapped. And
> since the sweeping of p2m table could be time consuming, it is done
> with hypercall continuation.
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I think ...

> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1031,6 +1031,35 @@ void p2m_change_type_range(struct domain *d,
>      p2m_unlock(p2m);
>  }
>  
> +/* Synchronously modify the p2m type for a range of gfns from ot to nt. */
> +void p2m_finish_type_change(struct domain *d,
> +                            gfn_t first_gfn, unsigned long max_nr,
> +                            p2m_type_t ot, p2m_type_t nt)
> +{
> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
> +    p2m_type_t t;
> +    unsigned long gfn = gfn_x(first_gfn);
> +    unsigned long last_gfn = gfn + max_nr - 1;
> +
> +    ASSERT(ot != nt);
> +    ASSERT(p2m_is_changeable(ot) && p2m_is_changeable(nt));
> +
> +    p2m_lock(p2m);
> +
> +    last_gfn = min(last_gfn, p2m->max_mapped_pfn);
> +    while ( gfn <= last_gfn )
> +    {
> +        get_gfn_query_unlocked(d, gfn, &t);
> +
> +        if ( t == ot )
> +            p2m_change_type_one(d, gfn, t, nt);
> +
> +        gfn++;
> +    }
> +
> +    p2m_unlock(p2m);
> +}

... this could be improved for reduction of overall latency: A fixed
limit as input if not very useful, as it does matter quite a bit whether
need to call p2m_change_type_one(). Doing 256 iteration with no
single call is going to be pretty fast I think, so it would be desirable
to weigh differently iterations with and without call to that function.

Jan


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