Re: [Xen-devel] Xen 4.1 + Linux compiled with PVH == BOOM

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.12.13 at 02:56, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> On Mon, Dec 23, 2013 at 11:49:49AM +0000, Jan Beulich wrote:
>> >>> "Jan Beulich" <jbeulich@xxxxxxxx> 12/23/13 10:39 AM >>>
>> >>>> Ian Campbell <Ian.Campbell@xxxxxxxxxx> 12/21/13 12:10 PM >>>
>> >>On Fri, 2013-12-20 at 12:57 -0500, Konrad Rzeszutek Wilk wrote:
>> >>> But perhaps that is not the way to do it and we should just cherry-pick
>> >>> 30832c06a8d1f9caff0987654ef9e24d59469d9a in Xen 4.1?
>> >>
>> >>I think we should do both, i.e. backport 30832c06a8d1 now to solve the
>> >>immediate problem and then look at fixing unstable to be more accepting
>> >>of new features which it doesn't yet know about.
>> >
>> >Hmm, not sure - without a split between necessary to be understood
>> >and acceptable to be unknown ones, I'm not sure either model will be
>> >the right thing.
>> 
>> And actually, in the case at hand the "BOOM" is correct: If the kernel tells
>> the hypervisor that it needs a feature the hypervisor doesn't even 
> recognize,
>> it's surely wrong to ignore this. The mistake here is for the kernel to 
> require
> 
> But it does not ignore it. It checks later on in construct_dom0 whether
> the 'required' parameters are present, like:

That's what I said: It is / would be wrong to ignore a feature th
kernel requires.

> if ( test_bit(XENFEAT_supervisor_mode_kernel, parms.f_required) )

This is even more confusing: 30832c06 is about hvm_callback_vector,
not supervisor_mode_kernel.

>> that feature statically in the first place - that should be done only if the 
> kernel
>> could _only_ boot in PVH mode.
> 
> The feature is not marked as "required" but rather - it can utilize said
> extension (so supported). I am advocating that the calleer checks that
> all of the required pieces are correct - and it can ignore the ones it
> has no idea off (which it does for some of the Xen ELF notes - ignores
> them if it has no idea of what they are).

What would be to point of telling the hypervisor that the kernel
can utilize a certain extension? The kernel could just utilize it, and
the hypervisor would know by that simple fact.

Jan


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