On 04/05/2021 18:44, Catalin Marinas wrote:

On Thu, Apr 29, 2021 at 05:06:07PM +0100, Steven Price wrote:

quoted

On 27/04/2021 18:58, Catalin Marinas wrote:

quoted

On Fri, Apr 16, 2021 at 04:43:08PM +0100, Steven Price wrote:

quoted

diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index 24223adae150..2b85a047c37d 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h

@@ -184,6 +184,20 @@ struct kvm_vcpu_events {
   	__u32 reserved[12];
   };
+struct kvm_arm_copy_mte_tags {
+	__u64 guest_ipa;
+	__u64 length;
+	union {
+		void __user *addr;
+		__u64 padding;
+	};
+	__u64 flags;
+	__u64 reserved[2];
+};

[...]

quoted

quoted

Maybe add the two reserved
values to the union in case we want to store something else in the
future.

I'm not sure what you mean here. What would the reserved fields be unioned
with? And surely they are no longer reserved in that case?

In case you want to keep the structure size the same for future
expansion and the expansion only happens via the union, you'd add some
padding in there just in case. We do this for struct siginfo with an
_si_pad[] array in the union.

Ah I see what you mean. In this case "padding" is just a sizer to ensure 
that flags is always the same alignment - it's not intended to be used. 
As I noted previously though it's completely pointless as this only on 
arm64 and even 32 bit Arm would naturally align the following __u64.

reserved[] is for expansion and I guess we could have a union over the 
whole struct (like siginfo) but I think it's generally clearer to just 
spell out the reserved fields at the end of the struct.

TLDR; the union will be gone along with "padding" in the next version. 
"reserved" remains at the end of the struct for future use.

Thanks,

Steve

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