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

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Message-ID: <f6a4587c-1db1-d477-5e6c-93dd603a11ec@linux.dev>
Date: Wed, 21 Feb 2024 00:30:24 +0800
From: Zenghui Yu <zenghui.yu@...ux.dev>
To: Oliver Upton <oliver.upton@...ux.dev>, kvmarm@...ts.linux.dev
Cc: 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

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?

>  	dist->lpi_list_count--;
>  
>  	kfree(irq);

Thanks,
Zenghui