RE: [PATCH v5 13/16] asm-generic/hyperv: introduce hv_device_id and auxiliary structures

From: Michael Kelley <mikelley@microsoft.com>
To: Wei Liu <wei.liu@kernel.org>, Arnd Bergmann <arnd@kernel.org>
Cc: Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	Linux Kernel List <linux-kernel@vger.kernel.org>,
	Vineeth Pillai <viremana@linux.microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Nuno Das Neves <nunodasneves@linux.microsoft.com>,
	"pasha.tatashin@soleen.com" <pasha.tatashin@soleen.com>,
	KY Srinivasan <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Arnd Bergmann <arnd@arndb.de>,
	"open list:GENERIC INCLUDE/ASM HEADER FILES" 
	<linux-arch@vger.kernel.org>
Subject: RE: [PATCH v5 13/16] asm-generic/hyperv: introduce hv_device_id and auxiliary structures
Date: Thu, 4 Feb 2021 16:46:14 +0000	[thread overview]
Message-ID: <MWHPR21MB1593E9C0DAF03C9B651D15FBD7B39@MWHPR21MB1593.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20210203140906.g35zr7366hh7p5f3@liuwe-devbox-debian-v2>

From: Wei Liu <wei.liu@kernel.org> Sent: Wednesday, February 3, 2021 6:09 AM
> 
> On Wed, Feb 03, 2021 at 02:49:53PM +0100, Arnd Bergmann wrote:
> > On Wed, Feb 3, 2021 at 2:26 PM Wei Liu <wei.liu@kernel.org> wrote:
> > > On Tue, Feb 02, 2021 at 05:02:48PM +0000, Wei Liu wrote:
> > > > On Tue, Jan 26, 2021 at 01:26:52AM +0000, Michael Kelley wrote:
> > > > > From: Wei Liu <wei.liu@kernel.org> Sent: Wednesday, January 20, 2021 4:01 AM
> > > > > > +union hv_device_id {
> > > > > > + u64 as_uint64;
> > > > > > +
> > > > > > + struct {
> > > > > > +         u64 :62;
> > > > > > +         u64 device_type:2;
> > > > > > + };
> > > > >
> > > > > Are the above 4 lines extraneous junk?
> > > > > If not, a comment would be helpful.  And we
> > > > > would normally label the 62 bit field as
> > > > > "reserved0" or something similar.
> > > > >
> > > >
> > > > No. It is not junk. I got this from a header in tree.
> > > >
> > > > I am inclined to just drop this hunk. If that breaks things, I will use
> > > > "reserved0".
> > > >
> > >
> > > It turns out adding reserved0 is required. Dropping this hunk does not
> > > work.
> >
> > Generally speaking, bitfields are not great for specifying binary interfaces,
> > as the actual bit order can differ by architecture. The normal way we get
> > around it in the kernel is to use basic integer types and define macros
> > for bit masks. Ideally, each such field should also be marked with a
> > particular endianess as __le64 or __be64, in case this is ever used with
> > an Arm guest running a big-endian kernel.
> 
> Thanks for the information.
> 
> I think we will need to wait until Microsoft Hypervisor clearly defines
> the endianess in its header(s) before we can make changes to the copy in
> Linux.
> 
> >
> > That said, if you do not care about the specific order of the bits, having
> > anonymous bitfields for the reserved members is fine, I don't see a
> > reason to name it as reserved.
> 
> Michael, let me know what you think. I'm not too fussed either way.
> 
> Wei.

I'm OK either way.  In the Hyper-V code we've typically given such
fields a name rather than leave them anonymous, which is why it stuck
out.

Michael

> 
> >
> >       Arnd