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

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Message-ID: <432193cf-e4bd-4cfb-a146-25b19c02d78e@amd.com>
Date: Mon, 26 May 2025 09:06:35 +0530
From: Sairaj Kodilkar <sarunkod@....com>
To: Sean Christopherson <seanjc@...gle.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 5/23/2025 8:03 PM, Sean Christopherson wrote:
> 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.

Makes sense, thanks for the explanation.

Thanks
Sairaj Kodilkar