Re: [Xen-devel] lock in vhpet

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

> -----Original Message-----
> From: Zhang, Yang Z
> Sent: Friday, April 27, 2012 9:25 AM
> To: andres@xxxxxxxxxxxxxxxx
> Cc: Tim Deegan; Keir Fraser; xen-devel@xxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> George.Dunlap@xxxxxxxxxxxxx
> Subject: RE: [Xen-devel] lock in vhpet
> 
> 
> > -----Original Message-----
> > From: Andres Lagar-Cavilla [mailto:andres@xxxxxxxxxxxxxxxx]
> > Sent: Friday, April 27, 2012 8:52 AM
> > To: Zhang, Yang Z
> > Cc: Tim Deegan; Keir Fraser; xen-devel@xxxxxxxxxxxxxxxxxxx;
> > olaf@xxxxxxxxx; George.Dunlap@xxxxxxxxxxxxx
> > Subject: RE: [Xen-devel] lock in vhpet
> >
> > >> -----Original Message-----
> > >> From: Tim Deegan [mailto:tim@xxxxxxx]
> > >> Sent: Friday, April 27, 2012 5:26 AM
> > >> To: Zhang, Yang Z
> > >> Cc: andres@xxxxxxxxxxxxxxxx; Keir Fraser;
> > >> xen-devel@xxxxxxxxxxxxxxxxxxx
> > >> Subject: Re: [Xen-devel] lock in vhpet
> > >>
> > >> At 02:36 +0000 on 25 Apr (1335321409), Zhang, Yang Z wrote:
> > >> > > > But actually, the first cs introduced this issue is 24770.
> > >> > > > When
> > >> > > > win8 booting and if hpet is enabled, it will use hpet as the
> > >> > > > time source and there have lots of hpet access and EPT
> > >> > > > violation. In EPT violation handler, it call
> > >> > > > get_gfn_type_access to get
> > the mfn.
> > >> > > > The cs 24770 introduces the gfn_lock for p2m lookups, and
> > >> > > > then the
> > >> issue
> > >> happens.
> > >> > > > After I removed the gfn_lock, the issue goes. But in latest
> > >> > > > xen, even I remove this lock, it still shows high cpu utilization.
> > >> > >
> > >> > > It would seem then that even the briefest lock-protected
> > >> > > critical section would cause this? In the mmio case, the p2m
> > >> > > lock taken in the hap fault handler is held during the actual
> > >> > > lookup, and for a couple of branch instructions afterwards.
> > >> > >
> > >> > > In latest Xen, with lock removed for get_gfn, on which lock is
> > >> > > time
> > >> spent?
> > >> > Still the p2m_lock.
> > >>
> > >> Can you please try the attached patch?  I think you'll need this
> > >> one plus the ones that take the locks out of the hpet code.
> > >>
> > >> This patch makes the p2m lock into an rwlock and adjusts a number
> > >> of the paths that don't update the p2m so they only take the read lock.
> > >> It's a bit rough but I can boot 16-way win7 guest with it.
> >
> > That is great news.
> >
> > Tim, thanks for the amazing work. I'm poring over the patch presently,
> > and will shoot at it with everything I've got testing-wise.
> >
> > I'm taking the liberty of pulling in Olaf (paging) and George (PoD) as
> > the changeset affects those subsystems.
> 
> But win8 guest shows BSOD with 32 VCPUs. :( The reason of BSOD is :
> SYSTEM_THREAD_EXCEPTION_NOT_HANDLED (ACPI.sys)
> 
Um....., I find this issue is related to xl not hypervisor.  
Will send a patch to fix it later.

yang



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