linux-kernel - Re: [PATCH v3 01/10] KVM: arm64: vgic: Store LPIs in an xarray

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Date: Wed, 21 Feb 2024 13:11:02 +0800
From: Zenghui Yu <yuzenghui@...wei.com>
To: Oliver Upton <oliver.upton@...ux.dev>
CC: Zenghui Yu <zenghui.yu@...ux.dev>, <kvmarm@...ts.linux.dev>,
	<kvm@...r.kernel.org>, Marc Zyngier <maz@...nel.org>, James Morse
	<james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 01/10] KVM: arm64: vgic: Store LPIs in an xarray

On 2024/2/21 1:15, Oliver Upton wrote:
> Hi Zenghui,
> 
> On Wed, Feb 21, 2024 at 12:30:24AM +0800, Zenghui Yu wrote:
>> On 2024/2/17 02:41, Oliver Upton wrote:
>>> Using a linked-list for LPIs is less than ideal as it of course requires
>>> iterative searches to find a particular entry. An xarray is a better
>>> data structure for this use case, as it provides faster searches and can
>>> still handle a potentially sparse range of INTID allocations.
>>>
>>> Start by storing LPIs in an xarray, punting usage of the xarray to a
>>> subsequent change.
>>>
>>> Signed-off-by: Oliver Upton <oliver.upton@...ux.dev>
>>
>> [..]
>>
>>> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
>>> index db2a95762b1b..c126014f8395 100644
>>> --- a/arch/arm64/kvm/vgic/vgic.c
>>> +++ b/arch/arm64/kvm/vgic/vgic.c
>>> @@ -131,6 +131,7 @@ void __vgic_put_lpi_locked(struct kvm *kvm, struct vgic_irq *irq)
>>>  		return;
>>>  	list_del(&irq->lpi_list);
>>> +	xa_erase(&dist->lpi_xa, irq->intid);
>>
>> We can get here *after* grabbing the vgic_cpu->ap_list_lock (e.g.,
>> vgic_flush_pending_lpis()/vgic_put_irq()).  And as according to vGIC's
>> "Locking order", we should disable interrupts before taking the xa_lock
>> in xa_erase() and we would otherwise see bad things like deadlock..
> 
> Nice catch!
> 
> Yeah, the general intention was to disable interrupts outside of the
> xa_lock, however:
> 
>> It's not a problem before patch #10, where we drop the lpi_list_lock and
>> start taking the xa_lock with interrupts enabled.  Consider switching to
>> use xa_erase_irq() instead?
> 
> I don't think this change is safe until #10, as the implied xa_unlock_irq()
> would re-enable interrupts before the lpi_list_lock is dropped. Or do I
> have wires crossed?

No, you're right.  My intention was to fix it in patch #10.  And as
you've both pointed out, using xa_erase_irq() can hardly be the correct
fix.  My mistake :-( .

Thanks,
Zenghui