Re: [PATCH 1/3] KVM: x86: Copy kvm_pmu_ops by value to eliminate layer of indirection

[Like Xu]

On 5/11/2021 11:30 pm, Sean Christopherson wrote:
> On Wed, Nov 03, 2021, Like Xu wrote:
> > Replace the kvm_pmu_ops pointer in common x86 with an instance of the
> > struct to save one pointer dereference when invoking functions. Copy the
> > struct by value to set the ops during kvm_init().
> >
> > Using kvm_x86_ops.hardware_enable to track whether or not the
> > ops have been initialized, i.e. a vendor KVM module has been loaded.
> >
> > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/pmu.c        | 41 +++++++++++++++++++++------------------
> >   arch/x86/kvm/pmu.h        |  4 +++-
> >   arch/x86/kvm/vmx/nested.c |  2 +-
> >   arch/x86/kvm/x86.c        |  3 +++
> >   4 files changed, 29 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 0772bad9165c..0db1887137d9 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -47,6 +47,9 @@
> >    *        * AMD:   [0 .. AMD64_NUM_COUNTERS-1] <=> gp counters
> >    */
> >   
> > +struct kvm_pmu_ops kvm_pmu_ops __read_mostly;
> > +EXPORT_SYMBOL_GPL(kvm_pmu_ops);
> > +
>
> ...
>
> > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > index b4ee5e9f9e20..1e793e44b5ff 100644
> > --- a/arch/x86/kvm/vmx/nested.c
> > +++ b/arch/x86/kvm/vmx/nested.c
> > @@ -4796,7 +4796,7 @@ void nested_vmx_pmu_entry_exit_ctls_update(struct kvm_vcpu *vcpu)
> >   		return;
> >   
> >   	vmx = to_vmx(vcpu);
> > -	if (kvm_x86_ops.pmu_ops->is_valid_msr(vcpu, MSR_CORE_PERF_GLOBAL_CTRL)) {
> > +	if (kvm_pmu_ops.is_valid_msr(vcpu, MSR_CORE_PERF_GLOBAL_CTRL)) {
>
> I would much prefer we export kvm_pmu_is_valid_msr() and go through that for nVMX
> than export all of kvm_pmu_ops for this one case.

Applied. Is it an abuse to export a function for only one case ?

> >   		vmx->nested.msrs.entry_ctls_high |=
> >   				VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
> >   		vmx->nested.msrs.exit_ctls_high |=
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ac83d873d65b..72d286595012 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11317,6 +11317,9 @@ int kvm_arch_hardware_setup(void *opaque)
> >   	memcpy(&kvm_x86_ops, ops->runtime_ops, sizeof(kvm_x86_ops));
> >   	kvm_ops_static_call_update();
> >   
> > +	if (kvm_x86_ops.hardware_enable)
>
> Huh?  Did you intend this to be?
>
> 	if (kvm_x86_ops.pmu_ops)
>
> Either way, I don't see the point, VMX and SVM unconditionally provide the ops.

Let me drop it.

> I would also say land this memcpy() above kvm_ops_static_call_update(), then the
> enabling patch can do the static call updates in kvm_ops_static_call_update()
> instead of adding another helper.

Both applied.

> > +		memcpy(&kvm_pmu_ops, kvm_x86_ops.pmu_ops, sizeof(kvm_pmu_ops));
>
> As part of this change, the pmu_ops should be moved to kvm_x86_init_ops and tagged
> as __initdata.  That'll save those precious few bytes, and more importantly make
> the original ops unreachable, i.e. make it harder to sneak in post-init modification
> bugs.

Applied. Opportunistically,

-struct kvm_pmu_ops [amd|intel]_pmu_ops = {
+struct kvm_pmu_ops [amd|intel]_pmu_ops __initdata = {

> > +
> >   	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
> >   		supported_xss = 0;
> >   
> > --
> > 2.33.0