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

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Message-ID: <ZdTeScN3XCgtRDJ9@linux.dev>
Date: Tue, 20 Feb 2024 17:15:53 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: Zenghui Yu <zenghui.yu@...ux.dev>
Cc: kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 01/10] KVM: arm64: vgic: Store LPIs in an xarray

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?

-- 
Thanks,
Oliver