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

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Date: Tue, 20 Feb 2024 17:43:03 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: Zenghui Yu <zenghui.yu@...ux.dev>, kvmarm@...ts.linux.dev,
	kvm@...r.kernel.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

On Tue, Feb 20, 2024 at 05:24:50PM +0000, Marc Zyngier wrote:
> On Tue, 20 Feb 2024 16:30:24 +0000,
> Zenghui Yu <zenghui.yu@...ux.dev> 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..
> > 
> > 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?
> 
> But does it actually work? xa_erase_irq() uses spin_lock_irq(),
> followed by spin_unlock_irq(). So if we were already in interrupt
> context, we would end-up reenabling interrupts. At least, this should
> be the irqsave version.

This is what I was planning to do, although I may kick it out to patch
10 to avoid churn.

> The question is whether we manipulate LPIs (in the get/put sense) on
> the back of an interrupt handler (like we do for the timer). It isn't
> obvious to me that it is the case, but I haven't spent much time
> staring at this code recently.

I think we can get into here both from contexts w/ interrupts disabled
or enabled. irqfd_wakeup() expects to be called w/ interrupts disabled.

All the more reason to use irqsave() / irqrestore() flavors of all of
this, and a reminder to go check all callsites that implicitly take the
xa_lock.

-- 
Thanks,
Oliver