Re: [PATCH 0/5] KVM: PPC: Book3S: Modules cleanup and unification

David Gibson <[email protected]> Fri, 03 Sep 2021 05:13:55 +0000
Newsgroups	org.kernel.vger.kvm-ppc,org.ozlabs.lists.linuxppc-dev
Message-ID	<YTGvE9o9e0qCz9xA@yekko>
On Thu, Sep 02, 2021 at 11:32:41AM -0300, Fabiano Rosas wrote:
> David Gibson <[email protected]> writes:
> 
> > On Wed, Sep 01, 2021 at 02:33:52PM -0300, Fabiano Rosas wrote:
> >> This series merges our three kvm modules kvm.ko, kvm-hv.ko and
> >> kvm-pr.ko into one kvm.ko module.
> >
> > That doesn't sound like a good idea to me.  People who aren't on BookS
> > servers don't want - and can't use - kvm-hv.  Almost nobody wants
> > kvm-pr.  It's also kind of inconsistent with x86, which has the
> > separate AMD and Intel modules.
> 
> But this is not altering the ability of having only kvm-hv or only
> kvm-pr. I'm taking the Kconfig options that used to produce separate
> modules and using them to select which code gets built into the one
> kvm.ko module.

> 
> Currently:
> 
> CONFIG_KVM_BOOK3S_64=m     <-- produces kvm.ko
> CONFIG_KVM_BOOK3S_64_HV=m  <-- produces kvm-hv.ko
> CONFIG_KVM_BOOK3S_64_PR=m  <-- produces kvm-pr.ko
> 
> I'm making it so we now have one kvm.ko everywhere, but there is still:
> 
> CONFIG_KVM_BOOK3S_64=m           <-- produces kvm.ko
> CONFIG_KVM_BOOK3S_HV_POSSIBLE=y  <-- includes HV in kvm.ko
> CONFIG_KVM_BOOK3S_PR_POSSIBLE=y  <-- includes PR in kvm.ko
> 
> In other words, if you are going to have at least two modules loaded at
> all times (kvm + kvm-hv or kvm + kvm-pr), why not put all that into one
> module? No one needs to build code they are not going to use, this is
> not changing.

Ah.. I see, you're removing the runtime switch from one to the other
at the same time as having just a single one loaded, but leaving the
ability to compile time switch.  And compile time is arguably good
enough for the cases I've described.

Ok, I see your point.

I still think it's conceptually not ideal, but the practical benefit
is more important.  Objection withdrawn.


> About consistency with x86, this situation is not analogous because we
> need to be able to load both modules at the same time, which means
> kvm.ko needs to stick around when one module goes away in case we want
> to load the other module. The KVM common code states that it expects to
> have at most one implementation:
> 
>         /*
>          * kvm_arch_init makes sure there's at most one caller
>          * for architectures that support multiple implementations,
>          * like intel and amd on x86.
>          (...)
> 
> which is not true in our case due to this requirement of having two
> separate modules loading independently.
> 
> (tangent) We are already quite different from other architectures since
> we're not making use of kvm_arch_init and some other KVM hooks, such as
> kvm_arch_check_processor_compat. So while other archs have their init
> dispatched by kvm common code, our init and cleanup happens
> independently in the ppc-specific modules, which obviously works but is
> needlessly different and has subtleties in the ordering of operations
> wrt. the kvm common code. (tangent)
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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