Re: [PATCH v3 00/14] x86/hvm: {svm,vmx} {c,h} cleanup

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.02.2023 08:09, Xenia Ragiadakou wrote:
> 
> On 2/27/23 14:17, Jan Beulich wrote:
>> On 27.02.2023 13:06, Andrew Cooper wrote:
>>> On 27/02/2023 11:33 am, Jan Beulich wrote:
>>>> On 27.02.2023 12:15, Andrew Cooper wrote:
>>>>> On 27/02/2023 10:46 am, Jan Beulich wrote:
>>>>>> On 24.02.2023 22:33, Andrew Cooper wrote:
>>>>>>> But I think we want to change tact slightly at this point, so I'm not
>>>>>>> going to do any further tweaking on commit.
>>>>>>>
>>>>>>> Next, I think we want to rename asm/hvm/svm/svm.h to asm/hvm/svm.h,
>>>>>>> updating the non-SVM include paths as we go.  Probably best to
>>>>>>> chain-include the other svm/hvm/svm/*.h headers temporarily, so we've
>>>>>>> only got one tree-wide cleanup of the external include paths.
>>>>>>>
>>>>>>> Quick tangent - I will be making all of that cpu_has_svm_*
>>>>>>> infrastructure disappear by moving it into the normal CPUID handling,
>>>>>>> but I've not had sufficient time to finish that yet.
>>>>>>>
>>>>>>> Next, hvm/svm/nestedsvm.h can merge straight into hvm/svm.h, and
>>>>>>> disappear (after my decoupling patch has gone in).
>>>>>> Why would you want to fold hvm/svm/nestedsvm.h into hvm/svm/svm.h?
>>>>>> The latter doesn't use anything from the former, does it?
>>>>> It's about what else uses them.
>>>>>
>>>>> hvm_vcpu needs both svm_vcpu and nestedsvm, so both headers are always
>>>>> included in tandem.
>>>> Well, yes, that's how things are today. But can you explain to me why
>>>> hvm_vcpu has to know nestedsvm's layout?
>>>
>>> Because it's part of the same single memory allocation.
>>
>> Which keeps growing, and sooner or later we'll need to find something
>> again to split off, so we won't exceed a page in size. The nested
>> structures would, to me, look to be prime candidates for such.
>>
>>>> If the field was a pointer,
>>>> a forward decl of that struct would suffice, and any entity in the
>>>> rest of Xen not caring about struct nestedsvm would no longer see it
>>>> (and hence also no longer be re-built if a change is made there).
>>>
>>> Yes, you could hide it as a pointer.  The cost of doing so is an
>>> unnecessary extra memory allocation, and extra pointer handling on
>>> create/destroy, not to mention extra pointer chasing in memory during use.
>>>
>>>>> nestedsvm is literally just one struct now, and no subsystem ought to
>>>>> have multiple headers when one will do.
>>>> When one will do, yes. Removing build dependencies is a good reason
>>>> to have separate headers, though.
>>>
>>> Its not the only only option, and an #ifdef CONFIG_NESTED_VIRT inside
>>> the struct would be an equally acceptable way of doing this which
>>> wouldn't involve making an extra memory allocation.
>>
>> That would make it a build-time decision, but then on NESTED_VIRT=y
>> hypervisors there might still be guests not meaning to use that
>> functionality, and for quite some time that may actually be a majority.
>>
>>> Everything you've posed here is way out of scope for Xenia's series.
>>
>> There was never an intention to extend the scope of the work she's doing.
>> Instead I was trying to limit the scope by suggesting to avoid a piece
>> of rework which later may want undoing.
> 
> Can I suggest to leave hvm/svm/svm.h and hvm/svm/nestedsvm.h separate 
> for now?

As per before - that's my preference. It'll be Andrew who you would need
to convince, as he did suggest the folding.

Jan