Re: [Xen-devel] [PATCH v10 5/6] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 4/5/2017 10:41 PM, George Dunlap wrote:
> On Sun, Apr 2, 2017 at 1:24 PM, Yu Zhang <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
> > asynchronously with the current p2m_change_entry_type_global()
> > interface.
> >
> > New field entry_count is introduced in struct p2m_domain, to record
> > the number of p2m_ioreq_server p2m page table entries. One nature of
> > these entries is that they only point to 4K sized page frames, because
> > all p2m_ioreq_server entries are originated from p2m_ram_rw ones in
> > p2m_change_type_one(). We do not need to worry about the counting for
> > 2M/1G sized pages.
> Assuming that all p2m_ioreq_server entries are *created* by
> p2m_change_type_one() may valid, but can you assume that they are only
> ever *removed* by p2m_change_type_one() (or recalculation)?
>
> What happens, for instance, if a guest balloons out one of the ram
> pages?  I don't immediately see anything preventing a p2m_ioreq_server
> page from being ballooned out, nor anything on the
> decrease_reservation() path decreasing p2m->ioreq.entry_count.  Or did
> I miss something?
>
> Other than that, only one minor comment...
Thanks for your thorough consideration, George. But I do not think we need to 
worry about this:

If the emulation is in process, the balloon driver cannot get a 
p2m_ioreq_server page - because
it is already allocated.

And even when emulation is finished, the balloon driver successfully get this 
page, and triggers
decrease_reservation, the purpose is to remove the current mapping relation 
between the gfn
and mfn in p2m. So IIUC, p2m_remove_page() will be triggered if everything is 
goes fine, and then
p2m_set_entry(), which will trigger the recalc logic eventually, either in 
ept_set_entry() or
p2m_pt_set_entry(). Then the entry_count will be updated in the recalc logic.

> > diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
> > index a57b385..6ec950a 100644
> > --- a/xen/arch/x86/mm/hap/hap.c
> > +++ b/xen/arch/x86/mm/hap/hap.c
> > @@ -187,6 +187,15 @@ out:
> >    */
> >   static int hap_enable_log_dirty(struct domain *d, bool_t log_global)
> >   {
> > +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
> > +
> > +    /*
> > +     * Refuse to turn on global log-dirty mode if
> > +     * there's outstanding p2m_ioreq_server pages.
> Grammar nit: if *there are* outstanding pages.
Oh, right. Thanks

B.R.
Yu
>   -George

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