Re: [Xen-devel] [PATCH v2 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server

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

On 4/11/2016 7:15 PM, Yu, Zhang wrote:
> On 4/8/2016 7:01 PM, George Dunlap wrote:
> > On 08/04/16 11:10, Yu, Zhang wrote:
> > [snip]
> > > BTW, I noticed your reply has not be CCed to mailing list, and I also
> > > wonder if we should raise this last question in community?
> > Oops -- that was a mistake on my part.  :-)  I appreciate the
> > discretion; just so you know in the future, if I'm purposely changing
> > the CC list (removing xen-devel and/or adding extra people), I'll almost
> > always say so at the top of the mail.
> >
> > > > And then of course there's the p2m_ioreq_server -> p2m_ram_logdirty
> > > > transition -- I assume that live migration is incompatible with this
> > > > functionality?  Is there anything that prevents a live migration from
> > > > being started when there are outstanding p2m_ioreq_server entries?
> > > Another good question, and the answer is unfortunately yes. :-)
> > >
> > > If live migration happens during the normal emulation process, entries
> > > marked with p2m_ioreq_server will be changed to p2m_log_dirty in
> > > resolve_misconfig(), and later write operations will change them to
> > > p2m_ram_rw, thereafter these pages can not be forwarded to device model.
> > >  From this point of view, this functionality is incompatible with live
> > > migration.
> > >
> > > But for XenGT, I think this is acceptable, because, if live migration
> > > is to be supported in the future, intervention from backend device
> > > model will be necessary. At that time, we can guarantee from the device
> > > model side that there's no outdated p2m_ioreq_server entries, hence no
> > > need to reset the p2m type back to p2m_ram_rw(and do not include
> > > p2m_ioreq_server in the P2M_CHANGEABLE_TYPES). By "outdated", I mean
> > > when an ioreq server is detached from p2m_ioreq_server, or before an
> > > ioreq server is attached to this type, entries marked with
> > > p2m_ioreq_server should be regarded as outdated.
> > >
> > > Is this acceptible to you? Any suggestions?
> > So the question is, as of this series, what happens if someone tries to
> > initiate a live migration while there are outstanding p2m_ioreq_server
> > entries?
> >
> > If the answer is "the ioreq server suddenly loses all control of the
> > memory", that's something that needs to be changed.
> Sorry, for this patch series, I'm afraid the above description is the
> answer.
>
> Besides, I find it's hard to change current code to both support the
> deferred resetting of p2m_ioreq_server and the live migration at the
> same time. One reason is that a page with p2m_ioreq_server behaves
> differently in different situations.
>
> My assumption of XenGT is that, for live migration to work, the device
> model should guarantee there's no outstanding p2m_ioreq_server pages
> in hypervisor(no need to use the deferred recalculation), and it is our
> device model who should be responsible for the copying of the write
> protected guest pages later.
>
> And another solution I can think of: when unmapping the ioreq server,
> we walk the p2m table and reset entries with p2m_ioreq_server back
> directly, instead of deferring the reset. And of course, this means
> performance impact. But since the mapping and unmapping of an ioreq
> server is not a frequent one, the performance penalty may be acceptable.
> How do you think about this approach?
George, sorry to bother you. Any comments on above option? :)

Another choice might be to let live migration fail if there's
outstanding p2m_ioreq_server entries. But I'm not quite inclined to do
so, because:
1> I'd still like to keep live migration feature for XenGT.
2> Not easy to know if there's outstanding p2m_ioreq_server entries. I
mean, since p2m type change is not only triggered by hypercall, to keep
a counter for remaining p2m_ioreq_server entries means a lot code
changes;

Besides, I wonder whether the requirement to reset the p2m_ioreq_server
is indispensable, could we let the device model side to be responsible
for this? The worst case I can imagine for device model failing to do
so is that operations of a gfn might be delivered to a wrong device
model. I'm not clear what kind of damage would this cause to the
hypervisor or other VM.

Does any other maintainers have any suggestions?
Thanks in advance! :)
> > If the answer is, "everything just works", that's perfect.
> >
> > If the answer is, "Before logdirty mode is set, the ioreq server has the
> > opportunity to detach, removing the p2m_ioreq_server entries, and
> > operating without that functionality", that's good too.
> >
> > If the answer is, "the live migration request fails and the guest
> > continues to run", that's also acceptable.  If you want this series to
> > be checked in today (the last day for 4.7), this is probably your best
> > bet.
> >
> >   -George
Regards
Yu

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