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 16:51, Boris Ostrovsky wrote:
> 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).

Right, but that can currently be enforced with an "if ( arg != 0 && arg
!= ~0 ) return -EOPNOTSUPP;" in the hypercall handler for now.

It still leaves us with the ability to add in LAPIC emulation in the
future by changing the auditing.  A blanket "no emulation" boolean is
very much harder to relax in the future.

~Andrew


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