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Re: [Xen-devel] [PATCH v11 10/34] ARM: GIC: export and extend vgic_init_pending_irq()



Hi Andre,

On 06/14/2017 04:54 PM, Andre Przywara wrote:
Hi,

On 12/06/17 16:36, Julien Grall wrote:
Hi Andre,

On 09/06/17 18:41, Andre Przywara wrote:
For LPIs we later want to dynamically allocate struct pending_irqs.
So beside needing to initialize the struct from there we also need
to clean it up and re-initialize it later on.
Export vgic_init_pending_irq() and extend it to be reusable.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
  xen/arch/arm/vgic.c        | 4 +++-
  xen/include/asm-arm/vgic.h | 1 +
  2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
index 2e4820f..7e8dba6 100644
--- a/xen/arch/arm/vgic.c
+++ b/xen/arch/arm/vgic.c
@@ -60,8 +60,10 @@ struct vgic_irq_rank *vgic_rank_irq(struct vcpu *v,
unsigned int irq)
      return vgic_get_rank(v, rank);
  }

-static void vgic_init_pending_irq(struct pending_irq *p, unsigned int
virq)
+void vgic_init_pending_irq(struct pending_irq *p, unsigned int virq)
  {
+    memset(p, 0, sizeof(*p));

So for initialization, we will clear the memory twice which looks rather
pointless (see the current caller).

We probably to drop the memset or replace xzalloc by xalloc in the
caller. I would be ok to see this change in a follow-up patch. Assuming
you will sent a patch:

So I checked the callers and now moved the memset from here to
its_discard_event(), just before the call to vgic_init_pending_irq().
That should be safe, because:
1) For the existing code (initialising SGIs/PPIs and SPIs) we always
zero pending_irq anyway, either by xzalloc or by an explicit memset.
2) The call in its_discard_event() has now an explicit memset before the
call.
3) Allocating struct pending_irqs for LPI upon mapping a device already
uses xzalloc, so they are initially zeroed. Before we re-use a struct,
we call its_discard_event(), which zeroes it as described in 2)

The place I am the most concerned is in the MAPTI. Because you would call vgic_init_pending_irq assuming this would have already been zeroed. It is not straight-forward when looking at the code who did that.

I would prefer to keep the memset in vgic_init_pending_irq and avoid it in the caller. This is more future proof.

So I merged the change (remove memset here, put it in
its_discard_event()) into the new series.
Please tell me if that is too dangerous and I can back it out again.
Let's look for a follow-up patch and not in this series. I don't want to delay the series just for that.

Cheers,

--
Julien Grall

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