Re: RE: Reducing I/O introduced domain scheduling

[Tim Deegan <Tim.Deegan@citrix.com>]

At 10:30 +0100 on 12 Oct (1286879457), Paul Durrant wrote:
> My concern is a read from a non-MMIO page following a write to an MMIO page.

Yes, buffering MMIO in the general case is totally unsafe.  That's why
the existing buffered-MMIO ring is only used for VGA.

Tim.

>   Paul
> 
> > -----Original Message-----
> > From: Keir Fraser [mailto:keir.xen@gmail.com] On Behalf Of Keir
> > Fraser
> > Sent: 12 October 2010 10:19
> > To: Paul Durrant; Dong, Eddie
> > Cc: xen-devel@lists.xensource.com; Zhang, Xiantao
> > Subject: Re: [Xen-devel] RE: Reducing I/O introduced domain
> > scheduling
> > 
> > No, you can't  vmexit on a fence. I don't know whether that matters,
> > so long
> > as buffered writes get flushed before the guest can observe their
> > effects
> > (presumably via some kind of I/O read). Agree that generalising the
> > buffered
> > I/O concept feels a bit dodgy however.
> > 
> >  -- Keir
> > 
> > On 12/10/2010 10:15, "Paul Durrant" <Paul.Durrant@citrix.com> wrote:
> > 
> > > Just wondering... does Xen/can Xen take VM exits on fences? If not
> > then I
> > > don't see you could safely buffer MMIO writes.
> > >
> > >   Paul
> > >
> > >> -----Original Message-----
> > >> From: xen-devel-bounces@lists.xensource.com [mailto:xen-devel-
> > >> bounces@lists.xensource.com] On Behalf Of Dong, Eddie
> > >> Sent: 12 October 2010 02:12
> > >> To: Keir Fraser
> > >> Cc: xen-devel@lists.xensource.com; Dong, Eddie; Zhang, Xiantao
> > >> Subject: [Xen-devel] Reducing I/O introduced domain scheduling
> > >>
> > >> Keir:
> > >> When running vConsolidation on top of Xen in  a 4-core
> > >> platform, we noticed the I/O introduced scheduling per CPU is ~3K
> > >> Hz, which seems to be too frequent and cause frequent involve of
> > >> domain 0 / Qemu, which may polute cache of the guest and thus
> > >> increase CPI (cycle per instruction).
> > >>
> > >> We are thinking if we can reduce the domin switch here, and
> > >> think the output of I/O can be buffered and return immediately.
> > The
> > >> buffered I/O can be flushed out at next IN emulation (or any
> > >> Hypervisor emulated I/O) or timeout such as 10 or 100 us to
> > >> guarantee minimal response.
> > >>
> > >> Ideally it can cover both PIO & MMIO, but we can start from
> > >> PIO.
> > >>
> > >> How do you think of that?
> > >>
> > >> Thx, Eddie
> > >> _______________________________________________
> > >> Xen-devel mailing list
> > >> Xen-devel@lists.xensource.com
> > >> http://lists.xensource.com/xen-devel
> > >
> > > _______________________________________________
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> > > Xen-devel@lists.xensource.com
> > > http://lists.xensource.com/xen-devel
> > 
> 
> 
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-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)