Re: [PATCH v3 4/4] io_uring: add support for zone-append

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jul 10, 2020 at 02:10:54PM +0100, Christoph Hellwig wrote:
> On Fri, Jul 10, 2020 at 12:35:43AM +0530, Kanchan Joshi wrote:
> > Append required special treatment (conversion for sector to bytes) for io_uring.
> > And we were planning a user-space wrapper to abstract that.
> > 
> > But good part (as it seems now) was: append result went along with cflags at
> > virtually no additional cost. And uring code changes became super clean/minimal
> > with further revisions.
> > While indirect-offset requires doing allocation/mgmt in application,
> > io-uring submission
> > and in completion path (which seems trickier), and those CQE flags
> > still get written
> > user-space and serve no purpose for append-write.
> 
> I have to say that storing the results in the CQE generally make
> so much more sense.  I wonder if we need a per-fd "large CGE" flag
> that adds two extra u64s to the CQE, and some ops just require this
> version.

If we're going to go the route of changing the CQE, how about:

 struct io_uring_cqe {
         __u64   user_data;      /* sqe->data submission passed back */
-        __s32   res;            /* result code for this event */
-        __u32   flags;
+	union {
+		struct {
+		        __s32   res;            /* result code for this event */
+		        __u32   flags;
+		};
+		__s64	res64;
+	};
 };

then we don't need to change the CQE size and it just depends on the SQE
whether the CQE for it uses res+flags or res64.