linux-kernel - Re: [PATCH] nvme: change nvme_passthru

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Date:   Mon, 4 Nov 2019 16:29:16 +0100
From:   Christoph Hellwig <hch@....de>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Marta Rybczynska <mrybczyn@...ray.eu>,
        Charles Machalow <csm10495@...il.com>,
        Christoph Hellwig <hch@....de>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        axboe <axboe@...com>, Sagi Grimberg <sagi@...mberg.me>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme: change nvme_passthru_cmd64's result field.

On Tue, Nov 05, 2019 at 12:01:51AM +0900, Keith Busch wrote:
> On Mon, Nov 04, 2019 at 03:56:57PM +0100, Marta Rybczynska wrote:
> > ----- On 4 Nov, 2019, at 15:51, Charles Machalow csm10495@...il.com wrote:
> > 
> > > For this one yes, UAPI size changes. Though I believe this IOCTL
> > > hasn't been in a released Kernel yet (just RC). Technically it may be
> > > changeable as a fix until the next Kernel is released. I do think its
> > > a useful enough
> > > change to warrant a late fix.
> > 
> > The old one is in UAPI for years. The new one is not yet, right. I'm OK
> > to change the new structure. To have compatibility you would have to use
> > the new structure (at least its size) in the user space code. This is
> > what you'd liek to do?
> 
> Charles is proposing only to modify the recently introduced 64-bit ioctl
> struct without touching the existing 32 bit version. He just wanted the
> lower 32-bits of the 64-bit result to occupy the same space as the 32-bit
> ioctl's result. That space in the 64-bit version is currently occupied
> by an implicit struct padding.

Except on 32-bit x86, which does not have the padding.  Which is why
the current layout is so bad, as it breaks 32-it x86 compat.