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

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Message-ID: <Z9h-ADGIb7B8no50@google.com>
Date: Mon, 17 Mar 2025 19:54:40 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Add a module param to control and enumerate
 device posted IRQs

On Mon, Mar 17, 2025 at 12:43:53PM -0700, Sean Christopherson wrote:
> 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@...gle.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.

I can understand a consistency argument, so I am fine either way too.
The main reason I suggested this is that it took me a second to realize
this is the same thing on both sides of the assignment.