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From:   Dexuan Cui <decui@microsoft.com>
To:     Greg KH <gregkh@linuxfoundation.org>,
        Nuno Das Neves <nunodasneves@linux.microsoft.com>
CC:     Wei Liu <wei.liu@kernel.org>,
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Subject: RE: [PATCH v4 13/15] uapi: hyperv: Add mshv driver headers defining
 hypervisor ABIs
Thread-Topic: [PATCH v4 13/15] uapi: hyperv: Add mshv driver headers defining
 hypervisor ABIs
Thread-Index: AQHZ9olsSlNFxyIgkE+OuXJhoX5gRLA55Dmw
Date:   Wed, 4 Oct 2023 17:36:32 +0000
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> From: Greg KH <gregkh@linuxfoundation.org>
> Sent: Tuesday, October 3, 2023 11:10 PM
> [...]
> On Tue, Oct 03, 2023 at 04:37:01PM -0700, Nuno Das Neves wrote:
> > On 9/30/2023 11:19 PM, Greg KH wrote:
> > > On Sat, Sep 30, 2023 at 10:01:58PM +0000, Wei Liu wrote:
> > > > On Sat, Sep 30, 2023 at 08:09:19AM +0200, Greg KH wrote:
> > > > > On Fri, Sep 29, 2023 at 11:01:39AM -0700, Nuno Das Neves wrote:
> > > > > > +/* Define connection identifier type. */
> > > > > > +union hv_connection_id {
> > > > > > +   __u32 asu32;
> > > > > > +   struct {
> > > > > > +           __u32 id:24;
> > > > > > +           __u32 reserved:8;
> > > > > > +   } __packed u;

IMO the "__packed" is unnecessary.

> > > > > bitfields will not work properly in uapi .h files, please never d=
o that.
> > > >
> > > > Can you clarify a bit more why it wouldn't work? Endianess? Alignme=
nt?
> > >
> > > 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 rea=
lly
> > > should as of course, this will not work properly with different endia=
n
> > > systems or many compilers.
> >
> > Yes, in
> https://docs.k/
> ernel.org%2Fdriver-
> api%2Fioctl.html&data=3D05%7C01%7Cdecui%40microsoft.com%7Ce404769e0f
> 85493f0aa108dbc4a08a27%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C
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> &reserved=3D0 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.

If it's hard to avoid bitfield here, maybe we can refer to the definition o=
f
struct iphdr in include/uapi/linux/ip.h

Thanks,
Dexuan