linux-kernel - Re: [PATCH v4 1/1] virt: acrn: Mark the uuid field as unused

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Message-ID: <Y3UKFQJJI/02QDWU@smile.fi.intel.com>
Date:   Wed, 16 Nov 2022 18:04:37 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, Fei Li <fei1.li@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v4 1/1] virt: acrn: Mark the uuid field as unused

On Wed, Nov 16, 2022 at 04:20:08PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Nov 16, 2022 at 03:29:31PM +0200, Andy Shevchenko wrote:
> > On Wed, Nov 16, 2022 at 12:42:16PM +0100, Greg Kroah-Hartman wrote:
> > > On Wed, Nov 16, 2022 at 11:22:54AM +0200, Andy Shevchenko wrote:

...

> > > > - * @uuid:		UUID of the VM. Pass to hypervisor directly.
> > > > + * @uuid:		Reserved (used to be UUID of the VM)
> > > 
> > > If it's reserved, then don't you need to check for 0?
> > 
> > Reserved in a way that it may content something we just don't care about.
> 
> "reserved" in the kernel ioctls mean "must be 0 and we will test for it,
> otherwise this is an empty/useless field that can never be touched again
> in the future.
> 
> Please spell it out in detail as to if you can ever use this later on,
> and what the kernel will do (if anything) if it is set.
> 
> And if "the kernel ignores it" then that means these bytes are now
> "empty space never to be used again", right?

Right, I will fix this in v5.

...

> > > > +	__u8	uuid[16];
> > > 
> > > You just changed the type here, so what is that going to break in
> > > userspace that depended on this being of a structure type and now it's
> > > an array?
> > 
> > It's the same. The previous was hidden behind additional type level.
> 
> Same size, yes.  Same C structure definition, no.

It doesn't matter, see below.

> > > And no other kernel changes needed?  Shouldn't you warn if this field is
> > > set?
> > 
> > No, as pointed out in the commit message kernel never ever used this.
> 
> That does not mean that userspace tools never did, right?  You are
> changing the structure definition, what tool just broke?

The only tool has been amended like a year ago, so the answer is none.
The commit message has links to the commits in question that made that
amendment.

Maybe I should remove Fixes tags? In such case we will very much know
that no old tools will be run on the new kernel.

-- 
With Best Regards,
Andy Shevchenko