Re: [PATCH v4 13/15] uapi: hyperv: Add mshv driver headers defining hypervisor ABIs

[Greg KH]

On Wed, Oct 04, 2023 at 11:16:46AM -0700, Nuno Das Neves wrote:
> On 10/4/2023 10:50 AM, Greg KH wrote:
> > On Wed, Oct 04, 2023 at 05:36:32PM +0000, Dexuan Cui wrote:
> > > > From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > 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.
> > > 
> These *are* going to the hypervisor - we use these same definitions in
> our driver for the kernel/hypervisor API. This is so we don't have to
> maintain two separate definitions for user/kernel and kernel/hypervisor
> APIs.

So these fields are just pass-through from userspace to the hypervisor
and are not touched at all by the kernel?  If so, I hope the hypervisor
is doing some validation of the data :)

> > > 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 :)
> > 
> Greg, there is nothing making us use bitfields. It just makes the work
> of porting the hypervisor definitions to Linux easier - aided by the
> fact that in practice, all the compilers in our stack produce the same
> code for these.

"our stack" is not how Linux works, you have to write files that work
for all compilers here.

Just use a normal variable and define the bits in them with proper bit
shifts or masks and that will be portable everywhere.  This isn't rocket
science...

thanks,

greg k-h