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 4/19/2016 12:37 PM, Tian, Kevin wrote:
> > From: Yu, Zhang [mailto:yu.c.zhang@xxxxxxxxxxxxxxx]
> > Sent: Thursday, April 14, 2016 6:45 PM
> >
> > 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 model side that there's no outdated p2m_ioreq_server entries, hence no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 By "outdated", I mean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 p2m_ioreq_server should be regarded as outdated.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! :)
> I'm not sure how above is working. In pre-copy phase (where logdirty
> is concerned), the device model is still actively serving requests from
> guest, including initiating new write-protection requests. How can you
> guarantee draining of outstanding p2m_ioreq_server entries w/o
> actually freezing device model (while freezing device model means guest
> driver might be blocked with random errors)?
You are right, and I'm not suggesting to clear the p2m_ioreq_server
entries when live migration happens. My suggestion is that either we
guarantee there is no outstanding p2m_ioreq_server entries right after
the ioreq server is unbounded, or do not support live migration for
now.  :)

B.R.
Yu

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