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

[Xenia Ragiadakou <burzalodowa@xxxxxxxxx>]

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?
> Jan
--
Xenia