Subject: Re: [RFC PATCH 4/9] vringh: unify the APIs for all accessors

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Dec 2022 02:04:18 -0500

On Tue, Dec 27, 2022 at 11:25:26AM +0900, Shunsuke Mie wrote:
> Each vringh memory accessors that are for user, kern and iotlb has own
> interfaces that calls common code. But some codes are duplicated and that
> becomes loss extendability.
> 
> Introduce a struct vringh_ops and provide a common APIs for all accessors.
> It can bee easily extended vringh code for new memory accessor and
> simplified a caller code.
> 
> Signed-off-by: Shunsuke Mie <[email-masked]>
> ---
>  drivers/vhost/vringh.c | 667 +++++++++++------------------------------
>  include/linux/vringh.h | 100 +++---
>  2 files changed, 225 insertions(+), 542 deletions(-)
> 
> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> index aa3cd27d2384..ebfd3644a1a3 100644
> --- a/drivers/vhost/vringh.c
> +++ b/drivers/vhost/vringh.c
> @@ -35,15 +35,12 @@ static __printf(1,2) __cold void vringh_bad(const char *fmt, ...)
>  }
>  
>  /* Returns vring->num if empty, -ve on error. */
> -static inline int __vringh_get_head(const struct vringh *vrh,
> -				    int (*getu16)(const struct vringh *vrh,
> -						  u16 *val, const __virtio16 *p),
> -				    u16 *last_avail_idx)
> +static inline int __vringh_get_head(const struct vringh *vrh, u16 *last_avail_idx)
>  {
>  	u16 avail_idx, i, head;
>  	int err;
>  
> -	err = getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
> +	err = vrh->ops.getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
>  	if (err) {
>  		vringh_bad("Failed to access avail idx at %p",
>  			   &vrh->vring.avail->idx);

I like that this patch removes more lines of code than it adds.

However one of the design points of vringh abstractions is that they were
carefully written to be very low overhead.
This is why we are passing function pointers to inline functions -
compiler can optimize that out.

I think that introducing ops indirect functions calls here is going to break
these assumptions and hurt performance.
Unless compiler can somehow figure it out and optimize?
I don't see how it's possible with ops pointer in memory
but maybe I'm wrong.

Was any effort taken to test effect of these patches on performance?

Thanks!