linux-kernel - Re: [PATCH v5 1/3] KVM: x86: Isolate edge vs. level check in userspace I/O APIC route scanning

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Message-ID: <0abe7616-0ef4-4098-bf55-8003ab958067@intel.com>
Date: Fri, 11 Apr 2025 10:55:14 +1200
From: Kai Huang <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "xuyun_xy.xy@...ux.alibaba.com"
	<xuyun_xy.xy@...ux.alibaba.com>, "zijie.wei@...ux.alibaba.com"
	<zijie.wei@...ux.alibaba.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/3] KVM: x86: Isolate edge vs. level check in
 userspace I/O APIC route scanning

On 11/04/25 10:46, Sean Christopherson wrote:
> On Tue, Mar 04, 2025, Kai Huang wrote:
>> On Mon, 2025-03-03 at 17:33 -0800, Sean Christopherson wrote:
>>> Extract and isolate the trigger mode check in kvm_scan_ioapic_routes() in
>>> anticipation of moving destination matching logic to a common helper (for
>>> userspace vs. in-kernel I/O APIC emulation).
>>>
>>> No functional change intended.
>>>
>>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>>
>> Reviewed-by: Kai Huang <kai.huang@...el.com>
>>
>>> ---
>>>   arch/x86/kvm/irq_comm.c | 10 ++++++----
>>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
>>> index 8136695f7b96..866f84392797 100644
>>> --- a/arch/x86/kvm/irq_comm.c
>>> +++ b/arch/x86/kvm/irq_comm.c
>>> @@ -424,10 +424,12 @@ void kvm_scan_ioapic_routes(struct kvm_vcpu *vcpu,
>>>   
>>>   			kvm_set_msi_irq(vcpu->kvm, entry, &irq);
>>>   
>>> -			if (irq.trig_mode &&
>>> -			    (kvm_apic_match_dest(vcpu, NULL, APIC_DEST_NOSHORT,
>>> -						 irq.dest_id, irq.dest_mode) ||
>>> -			     kvm_apic_pending_eoi(vcpu, irq.vector)))
>>> +			if (!irq.trig_mode)
>>> +				continue;
>>
>> Perhaps take this chance to make it explicit?
>>
>> 			if (irq.trig_mode != IOAPIC_LEVEL_TRIG)
>> 				continue;
>>
>> kvm_ioapic_scan_entry() also checks against IOAPIC_LEVEL_TRIG explicitly.
> 
> Hmm, I'm leaning "no".  kvm_set_msi_irq() isn't I/O APIC specific (and obviously
> neither is "struct kvm_lapic_irq").  The fact that it sets irq.trig_mode to '0'
> or '1', and that the '1' value in particular happens to match IOAPIC_LEVEL_TRIG
> is somewhat of a coincidence.
> 
> kvm_ioapic_scan_entry() on the other operates on a "union kvm_ioapic_redirect_entry"
> object, in which case trig_mode is guaranteed to be '0' or '1', i.e. is exactly
> IOAPIC_EDGE_TRIG or IOAPIC_LEVEL_TRIG.
> 
> 	u8 trig_mode:1;

This makes sense.  Thanks for clarifying.

> 
> So as much as I advocate for consistency, I think in this case it makes sense to
> be consistent with __apic_accept_irq(), which only cares about zero vs. non-zero.

Yeah LGTM.