On Tue, Oct 03, 2023 at 04:37:01PM -0700, Nuno Das Neves wrote:

On 9/30/2023 11:19 PM, Greg KH wrote:

quoted

On Sat, Sep 30, 2023 at 10:01:58PM +0000, Wei Liu wrote:

quoted

On Sat, Sep 30, 2023 at 08:09:19AM +0200, Greg KH wrote:

quoted

On Fri, Sep 29, 2023 at 11:01:39AM -0700, Nuno Das Neves wrote:

quoted

+/* Define connection identifier type. */
+union hv_connection_id {
+	__u32 asu32;
+	struct {
+		__u32 id:24;
+		__u32 reserved:8;
+	} __packed u;

bitfields will not work properly in uapi .h files, please never do that.

Can you clarify a bit more why it wouldn't work? Endianess? Alignment?

Yes to both.

Did you all read the documentation for how to write a kernel api?  If
not, please do so.  I think it mentions bitfields, but it not, it really
should as of course, this will not work properly with different endian
systems or many compilers.

Yes, in https://docs.kernel.org/driver-api/ioctl.html it says that it is
"better to avoid" bitfields.

Unfortunately bitfields are used in the definition of the hypervisor
ABI. We import these definitions directly from the hypervisor code.

So why do you feel you have to use this specific format for your
user/kernel api?  That is not what is going to the hypervisor.

In practice the hypervisor, linux, and VMM compilers all produce the
same layout for bitfields on the architectures we support.

You are getting lucky, please do this properly, without a bitfield
marking in the structure.

thanks,

greg k-h

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