Re: [Xen-devel] [PATCH v3 3/9] xen/mm: move modify_identity_mmio to global file and drop __init

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

On Wed, Jun 21, 2017 at 05:57:19AM -0600, Jan Beulich wrote:
> >>> On 21.06.17 at 13:11, <roger.pau@xxxxxxxxxx> wrote:
> > On Fri, May 19, 2017 at 07:35:39AM -0600, Jan Beulich wrote:
> >> >>> On 27.04.17 at 16:35, <roger.pau@xxxxxxxxxx> wrote:
> >> > +int modify_mmio(struct domain *d, gfn_t gfn, mfn_t mfn, unsigned long 
> >> > nr_pages,
> >> > +                const bool map)
> >> > +{
> >> > +    int rc;
> >> > +
> >> > +    /*
> >> > +     * Make sure this function is only used by the hardware domain, 
> >> > because it
> >> > +     * can take an arbitrary long time, and could DoS the whole system.
> >> > +     */
> >> > +    ASSERT(is_hardware_domain(d));
> >> 
> >> If that can happen arbitrarily at run time (rather than just at boot,
> >> as suggested by the removal of __init), it definitely can't remain as
> >> is and will instead need to make use of continuations. I'm therefore
> >> unconvinced you really want to move this code instead of simply
> >> calling {,un}map_mmio_regions() while taking care of preemption
> >> needs.
> > 
> > I'm not sure I know how to use continuations with non-hypercall
> > vmexits. Do you have any recommendations about how to do this? pause
> > the domain and run the mmio changes inside of a tasklet?
> 
> That would be one option. Or you could derive from the approach
> used for waiting for a response from the device model.

AFAICT the ioreq code pauses the domain and waits for a reply from the
dm, but in that case I would still need the tasklet in order to perform
the work (since there's no dm here).

> Even exiting
> back to the guest without updating rIP may be possible, provided
> you have a means to store the continuation information such that
> when coming back you won't start from the beginning again.

I don't really fancy this since it would mean wasting a lot of time in
vmexits/vmenters.

Roger.

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