Re: [Xen-devel] [PATCH v2 11/22] xen/x86: allow disabling emulated devices for HVM guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/01/2015 11:46 AM, Andrew Cooper wrote:
> On 01/07/15 15:46, Roger Pau Monne wrote:
> > Introduce a new DOMCTL flag that can be used to disable device emulation
> > inside of Xen for HVM guests. The following emulated devices are disabled
> > when the XEN_DOMCTL_CDF_noemu is used: hpet, pmtimer, rtc, ioapic, lapic,
> > pic and pmu. Also all the MMIO handlers are disabled.
> >
> > Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > Cc: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> > Cc: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@xxxxxxx>
> > Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> > Cc: Eddie Dong <eddie.dong@xxxxxxxxx>
> > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> I would be hesitant to have a blanket change like this.
>
> Consider APICV/AVIC.  For performance reasons, we absolutely want HVM
> and PVH to make use of them, as they are substantially more efficient
> using hardware support than evening using plain evtchn hypercalls.
>
> However, the flipside is that we must provide an LAPIC emulation to
> cover the bits which hardware cannot virtualise.
>
> As a random idea, how about having a new hypercall or hvmparam which
> provides a bitmap of permitted emulators?  This would allow far finer
> grain control over what is and isn't available to a domain.
I think we also need to decide on which subsets of emulators we are 
going to support, otherwise test matrix will become pretty big. For 
example, initially we may want to allow all (for what we now call HVM) 
or none (PVH).

-boris

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