Re: [PATCH] KVM: x86: Add a module param to control and enumerate device posted IRQs

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 17, 2025, Yosry Ahmed wrote:
> On Fri, Mar 14, 2025 at 07:56:15PM -0700, Sean Christopherson wrote:
> > Add a module param to allow disabling device posted interrupts without
> > having to sacrifice all of APICv/AVIC, and to also effectively enumerate
> > to userspace whether or not KVM may be utilizing device posted IRQs.
> > Disabling device posted interrupts is very desirable for testing, and can
> > even be desirable for production environments, e.g. if the host kernel
> > wants to interpose on device interrupts.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h | 1 +
> >  arch/x86/kvm/svm/avic.c         | 3 +--
> >  arch/x86/kvm/vmx/posted_intr.c  | 7 +++----
> >  arch/x86/kvm/x86.c              | 9 ++++++++-
> >  4 files changed, 13 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index d881e7d276b1..bf11c5ee50cb 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1922,6 +1922,7 @@ struct kvm_arch_async_pf {
> >  extern u32 __read_mostly kvm_nr_uret_msrs;
> >  extern bool __read_mostly allow_smaller_maxphyaddr;
> >  extern bool __read_mostly enable_apicv;
> > +extern bool __read_mostly enable_device_posted_irqs;
> >  extern struct kvm_x86_ops kvm_x86_ops;
> >  
> >  #define kvm_x86_call(func) static_call(kvm_x86_##func)
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 65fd245a9953..e0f519565393 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -898,8 +898,7 @@ int avic_pi_update_irte(struct kvm *kvm, unsigned int host_irq,
> >  	struct kvm_irq_routing_table *irq_rt;
> >  	int idx, ret = 0;
> >  
> > -	if (!kvm_arch_has_assigned_device(kvm) ||
> > -	    !irq_remapping_cap(IRQ_POSTING_CAP))
> > +	if (!kvm_arch_has_assigned_device(kvm) || !enable_device_posted_irqs)
> 
> This function will now also be skipped if enable_apicv is false. Is this
> always the case here for some reason? Sorry if I missed something
> obvious.

Working as intended, though I failed to document it.  Hrm, but I wasn't expecting
this to be a functional change.  Oh, I know what happened.  I had originally
tacked this on to a big series to clean up the IRTE stuff (spoiler alert), and in
that series common code checked kvm_arch_has_irq_bypass() (which incorporates
enable_apicv) before calling pi_update_irte().

I'll prepend a patch or three to do minimal cleanup before introducing the new
module param.

> > @@ -9772,6 +9776,9 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> >  	if (r != 0)
> >  		goto out_mmu_exit;
> >  
> > +	enable_device_posted_irqs = enable_device_posted_irqs && enable_apicv &&
> > +				    irq_remapping_cap(IRQ_POSTING_CAP);
> 
> Maybe this is clearer:
> 
> 	enable_device_posted_irqs &= enable_avivc && irq_remapping_cap(IRQ_POSTING_CAP);

I don't have a strong opinion.  I went with the "self check" approach purely
because SVM does so for a few params, e.b.

	nrips = nrips && boot_cpu_has(X86_FEATURE_NRIPS);

Anyone else care either way?  If not, I'll go with Yosry's suggestion.