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Date:   Wed, 4 Oct 2023 19:50:31 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Dexuan Cui <decui@microsoft.com>
Cc:     Nuno Das Neves <nunodasneves@linux.microsoft.com>,
        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
Message-ID: <2023100415-diving-clapper-a2a7@gregkh>
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On Wed, Oct 04, 2023 at 05:36:32PM +0000, Dexuan Cui wrote:
> > 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 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.k/
> > ernel.org%2Fdriver-
> > api%2Fioctl.html&data=05%7C01%7Cdecui%40microsoft.com%7Ce404769e0f
> > 85493f0aa108dbc4a08a27%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C
> > 0%7C638319966071263290%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLj
> > AwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%
> > 7C%7C&sdata=RiLNA5DRviWBQK6XXhxC4m77raSDBb%2F0BB6BDpFPUJY%3D
> > &reserved=0 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 of
> struct iphdr in include/uapi/linux/ip.h

It is not hard to avoid using bitfields, just use the proper definitions
to make this portable for all compilers.  And ick, ip.h is not a good
thing to follow :)

thanks,

greg k-h