Re: [PATCH v5 2/3] xen/arm: move setting of new target vcpu to vgic_migrate_irq

[Julien Grall <julien.grall@arm.com>]

Hi Stefano,

On 01/03/17 22:15, Stefano Stabellini wrote:
> Move the atomic write of rank->vcpu, which sets the new vcpu target, to
> vgic_migrate_irq, at the beginning of the lock protected area (protected
> by the vgic lock).
>
> This code movement reduces race conditions between vgic_migrate_irq and
> setting rank->vcpu on one pcpu and gic_update_one_lr on another pcpu.
>
> When gic_update_one_lr and vgic_migrate_irq take the same vgic lock,
> there are no more race conditions with this patch. When vgic_migrate_irq
> is called multiple times while GIC_IRQ_GUEST_MIGRATING is already set, a
> race condition still exists because in that case gic_update_one_lr and
> vgic_migrate_irq take different vgic locks.
>
> Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
> ---
>  xen/arch/arm/vgic-v2.c     |  5 ++---
>  xen/arch/arm/vgic-v3.c     |  4 +---
>  xen/arch/arm/vgic.c        | 15 ++++++++++-----
>  xen/include/asm-arm/vgic.h |  3 ++-
>  4 files changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/xen/arch/arm/vgic-v2.c b/xen/arch/arm/vgic-v2.c
> index 0674f7b..43b4ac3 100644
> --- a/xen/arch/arm/vgic-v2.c
> +++ b/xen/arch/arm/vgic-v2.c
> @@ -158,10 +158,9 @@ static void vgic_store_itargetsr(struct domain *d, struct vgic_irq_rank *rank,
>          {
>              vgic_migrate_irq(d->vcpu[old_target],
>                               d->vcpu[new_target],
> -                             virq);
> +                             virq,
> +                             &rank->vcpu[offset]);
>          }
> -
> -        write_atomic(&rank->vcpu[offset], new_target);

With this change rank->vcpu[offset] will not be updated for virtual SPIs 
(e.g p->desc != NULL). And therefore affinity for them will not work.

However, from my understanding the problem you are trying to solve with 
this patch is having rank->vcpu[offset] to be set as soon as possible. 
It does not really matter if it is protected by the lock, what you care 
is rank->vcpu[offset] been seen before the lock has been released.

So if GIC_IRQ_GUEST_MIGRATE is set and gic_update_one_lr is running 
straight after the lock is released, rank->vcpu[offset] will contain the 
correct vCPU.

A better approach would be to move write_atomic(...) before 
vgic_migrated_irq(...). What do you think?

Cheers,

-- 
Julien Grall

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