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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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