linux-kernel - Re: [PATCH v3 13/13] KVM: selftests: Add a KVM_IRQFD test to verify uniqueness requirements

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Message-ID: <aDB-2lcq4jJm9-OV@google.com>
Date: Fri, 23 May 2025 07:33:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sairaj Kodilkar <sarunkod@....com>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, 
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, 
	Juergen Gross <jgross@...e.com>, Stefano Stabellini <sstabellini@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Shuah Khan <shuah@...nel.org>, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, linux-kernel@...r.kernel.org, 
	linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	K Prateek Nayak <kprateek.nayak@....com>, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v3 13/13] KVM: selftests: Add a KVM_IRQFD test to verify
 uniqueness requirements

On Fri, May 23, 2025, Sairaj Kodilkar wrote:
> On 5/23/2025 5:22 AM, Sean Christopherson wrote:
> 
> > +
> > +int main(int argc, char *argv[])
> > +{
> > +	pthread_t racing_thread;
> > +	int r, i;
> > +
> > +	/* Create "full" VMs, as KVM_IRQFD requires an in-kernel IRQ chip. */
> > +	vm1 = vm_create(1);
> > +	vm2 = vm_create(1);
> > +
> > +	WRITE_ONCE(__eventfd, kvm_new_eventfd());
> > +
> > +	kvm_irqfd(vm1, 10, __eventfd, 0);
> > +
> > +	r = __kvm_irqfd(vm1, 11, __eventfd, 0);
> > +	TEST_ASSERT(r && errno == EBUSY,
> > +		    "Wanted EBUSY, r = %d, errno = %d", r, errno);
> > +
> > +	r = __kvm_irqfd(vm2, 12, __eventfd, 0);
> > +	TEST_ASSERT(r && errno == EBUSY,
> > +		    "Wanted EBUSY, r = %d, errno = %d", r, errno);
> > +
> > +	kvm_irqfd(vm1, 11, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > +	kvm_irqfd(vm1, 12, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > +	kvm_irqfd(vm1, 13, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > +	kvm_irqfd(vm1, 14, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> 
> Hi Sean,
> I dont see any allocation for the GSI 13 and 14..
> Is there any reason for the deassigning these two GSIs ?

Yes, KVM's rather bizarre ABI is that DEASSIGN is allowed even if the VM doesn't
have a corresponding assigned irqfd.  The reason I added these early DEASSIGN
calls is so that there will be an easier-to-debug failure if KVM's behavior
changes (the racing threads part of the test abuses KVM's ABI).  I didn't add a
comment because the helpers already have comments, but looking at this again, I
agree that main() needs a better comment.