Hi Paul,

On 19/10/2020 08:23, Paul Durrant wrote:
-----Original Message-----
From: Julien Grall <[email protected]>
Sent: 16 October 2020 16:47
To: Paul Durrant <[email protected]>; [email protected]
Cc: Paul Durrant <[email protected]>; Daniel De Graaf 
<[email protected]>; Ian Jackson
<[email protected]>; Wei Liu <[email protected]>; Andrew Cooper 
<[email protected]>; George Dunlap
<[email protected]>; Jan Beulich <[email protected]>; Stefano Stabellini
<[email protected]>
Subject: Re: [PATCH 2/5] iommu / domctl: introduce XEN_DOMCTL_iommu_ctl

Hi Paul,

On 05/10/2020 10:49, Paul Durrant wrote:
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 791f0a2592..75e855625a 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -1130,6 +1130,18 @@ struct xen_domctl_vuart_op {
                                    */
   };

+/*
+ * XEN_DOMCTL_iommu_ctl
+ *
+ * Control of VM IOMMU settings
+ */
+
+#define XEN_DOMCTL_IOMMU_INVALID 0

I can't find any user of XEN_DOMCTL_IOMMU_INVALID. What's the purpose
for it?


It's just a placeholder. I think it's generally safer that a zero opcode value 
is invalid.

Thanks for the explanation. I first thought the goal would to somehow invalidate the IOMMU :).

Anyway, it might be worth adding /* Reserved - should never be used */ on top.

Cheers,

--
Julien Grall

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