Re: [PATCH] tap: Send frames after the first one in tap_send_frames_pasta()

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 13 Feb 2023 13:24:58 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Feb 13, 2023 at 02:12:11AM +0100, Stefano Brivio wrote:
> > ...instead of repeatedly sending out the first one in iov.
> > 
> > Fixes: e21ee41ac35a ("tcp: Combine two parts of pasta tap send path together")
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > ---
> > I just applied this, to unblock a series by David which was pending
> > for way too long. The commit reference in Fixes: refers to a commit
> > from said series which I'm pushing out together with this patch.  
> 
> Huh... how did this ever work even slightly.  From that point of view,
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
> > Posting anyway for reviews.  
> 
> That said..
> 
> > 
> >  tap.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tap.c b/tap.c
> > index af9bc15..716d887 100644
> > --- a/tap.c
> > +++ b/tap.c
> > @@ -316,12 +316,13 @@ static void tap_send_frames_pasta(struct ctx *c,
> >  {
> >  	size_t i;
> >  
> > -	for (i = 0; i < n; i++) {
> > +	for (i = 0; i < n; i++, iov++) {  
> 
> I quite dislike having multiple "counters" that need to be updated for
> each loop iteration (manual strength reduction.  It's really easy to
> make a mistake in later changes and let the two values get out of sync
> - which is exactly what I did with the earlier change that introduced
> this bug.

Um, yes. I try, whenever possible, to use just one "iterator", which
would be iov, but the price of doing that "cleanly" here is wasting a
struct iovec just to have a zero iov_len at the end, which makes little
sense.

> W.r.t. performance, I generally trust the compiler's automatic
> strength reduction to have a better idea of whether it will be worth
> it or not than my own guess.
> 
> >  		if (write(c->fd_tap, (char *)iov->iov_base, iov->iov_len) < 0) {  
> 
> So, my *intention* on the older patch was to replace 'iov->' above
> with 'iov[i].'

That would also be consistent with tap_send_frames_passt(), so sure,
let's change it. I can submit a patch too.

-- 
Stefano