Re: [PATCH 00/45] KVM: Refactor vCPU creation

Christoffer Dall
2019-10-22T11:51:52Z

Hi Sean,

On Mon, Oct 21, 2019 at 06:58:40PM -0700, Sean Christopherson wrote:
> *************************** DISCLAIMER **********************************
> The non-x86 arch specific patches are completely untested. Although the
> changes are conceptually straightforward, I'm not remotely confident that
> the patches are bug free, e.g. checkpatch caught several blatant typos
> that would break compilation.
> *************************************************************************
>
> The end goal of this series is to strip down the interface between common
> KVM code and arch specific code so that there is precisely one arch hook
> for creating a vCPU and one hook for destroying a vCPU. In addition to
> cleaning up the code base, simplifying the interface gives architectures
> more freedom to organize their vCPU creation code.
>
> KVM's vCPU creation code is comically messy. kvm_vm_ioctl_create_vcpu()
> calls three separate arch hooks: init(), create() and setup(). The init()
> call is especially nasty as it's hidden away in a common KVM function,
> kvm_init_vcpu(), that for all intents and purposes must be immediately
> invoked after the vcpu object is allocated.
>
> Not to be outdone, vCPU destruction also has three arch hooks: uninit(),
> destroy() and free(), the latter of which isn't actually invoked by common
> KVM code, but the hook declaration still exists because architectures are
> relying on its forward declaration.
>
> Eliminating the extra arch hooks is relatively straightforward, just
> tedious. For the most part, there is no fundamental constraint that
> necessitated the proliferation of arch hooks, rather they crept in over
> time, usually when x86-centric code was moved out of generic KVM and into
> x86 code.
>
> E.g. kvm_arch_vcpu_setup() was added to allow x86 to do vcpu_load(), which
> can only be done after preempt_notifier initialization, but adding setup()
> overlooked the fact that the preempt_notifier was only initialized after
> kvm_arch_vcpu_create() because preemption support was added when x86's MMU
> setup (the vcpu_load() user) was called from common KVM code.
>
> For all intents and purposes, there is no true functional change in this
> series. The order of some allocations will change, and a few memory leaks
> are fixed, but the actual functionality of a guest should be unaffected.
>
> Patches 01-03 are bug fixes in error handling paths that were found by
> inspection when refactoring the associated code.
>
> Patches 04-43 refactor each arch implementation so that the unwanted arch
> hooks can be dropped without a functional change, e.g. move code out of
> kvm_arch_vcpu_setup() so that all implementations are empty, then drop the
> functions and caller.
>
> Patches 44-45 are minor clean up to eliminate kvm_vcpu_uninit().
>
>
> The net result is to go from this:
>
> vcpu = kvm_arch_vcpu_create(kvm, id);
> |
> |-> kvm_vcpu_init()
> |
> |-> kvm_arch_vcpu_init()
>
> if (IS_ERR(vcpu)) {
> r = PTR_ERR(vcpu);
> goto vcpu_decrement;
> }
>
> preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops);
>
> r = kvm_arch_vcpu_setup(vcpu);
> if (r)
> goto vcpu_destroy;
>
> to this:
>
> r = kvm_arch_vcpu_precreate(kvm, id);
> if (r)
> goto vcpu_decrement;
>
> vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL);
> if (!vcpu) {
> r = -ENOMEM;
> goto vcpu_decrement;
> }
>
> page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> if (!page) {
> r = -ENOMEM;
> goto vcpu_free;
> }
> vcpu->run = page_address(page);
>
> kvm_vcpu_init(vcpu, kvm, id);
>
> r = kvm_arch_vcpu_create(vcpu);
> if (r)
> goto vcpu_free_run_page;
>

What a fantastically welcome piece of work! Thanks for doing this,
many's the time I waded through all those calls to ensure a patch was
doing the right thing.

Modulo the nit in patch 42, the arm64 changes survive a guest boot +
hackbench and build fine. The lack of changing the arm-specific destroy
function to a void also causes a series of warnings for a 32-bit arm
build, but otherwise builds fine.

You can add my:

Acked-by: Christoffer Dall <christoffer.dall@arm.com>

To the arm/arm64 and generic parts.

Thanks,

Christoffer