Re: [Xen-devel] [RFC PATCH 1/8]: PVH: Basic and preparatory changes

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Thu, Sep 13, 2012 at 2:08 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Thu, 2012-09-13 at 18:36 +0100, Pasi Kärkkäinen wrote:
>> On Thu, Sep 13, 2012 at 10:14:39AM -0700, Mukesh Rathor wrote:
>> > On Thu, 13 Sep 2012 07:00:07 +0100
>> > Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> > > The toolstack (or the dom0 builder) can make the determination whether
>> > > to enable this mode itself itself based on the notes (although it is
>> > > always handy to have an override to force things for debug).
>> >
>> > Hmmm... So you are suggesting that if hardware and xen supports it, then
>> > PV should just boot in PVH mode and user has no way to override it. I
>> > don't know if that's a good idea in the short term, perhaps in the long
>> > term. Let me discuss with folks here a bit.
>> >
>>
>> I read that as "if hardware, xen and kernel supports PVH, enable it as a 
>> default,
>> but still allow the user to force enable/disable it in the domain cfgfile 
>> for easier debugging".
>
> That's right. I have no objection to an override, in fact I think it's a
> requirement to be able to force this sort of feature on/off (to aid for
> debugging, account for differing workload requirements and allowing the
> test system to test both configurations etc).
>
> Long term the aim should be for the default to be on whenever possible.
> Maybe the default should remain off for the time being, or maybe we want
> to get as much exposure of PVH as possible ASAP, which would argue for
> it being on by default in unstable, perhaps with a decision being made
> around freeze time as to the default in 4.3.
>
> The feature declarations in the ELF note also allow the toolstack to
> sanity check what the user asked for and report an error if it isn't
> possible, rather than booting a kernel in a mode which cannot possibly
> work.

We can also punt the decision to the guest. Meaning it can have one of
those features flags saying: "I_WANT_PVH"

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