Re: [Qemu-devel] [PATCH 2/3] xen: slightly simplify bufioreq handling

[Stefano Stabellini]

On Wed, 23 Nov 2016, Jan Beulich wrote:
> There's no point setting fields always receiving the same value on each
> iteration, as handle_ioreq() doesn't alter them anyway. Set state and
> count once ahead of the loop, drop the redundant clearing of
> data_is_ptr, and avoid the meaningless setting of df altogether.

Why setting df is meaningless?


> Also avoid doing an unsigned long calculation of size when the field to
> be initialized is only 32 bits wide (and the shift value in the range
> 0...3).
> 
> Signed-off-by: Jan Beulich <address@hidden>
> 
> --- a/xen-hvm.c
> +++ b/xen-hvm.c
> @@ -995,6 +995,8 @@ static int handle_buffered_iopage(XenIOS
>      }
>  
>      memset(&req, 0x00, sizeof(req));
> +    req.state = STATE_IOREQ_READY;
> +    req.count = 1;
>  
>      for (;;) {
>          uint32_t rdptr = buf_page->read_pointer, wrptr;
> @@ -1009,15 +1011,11 @@ static int handle_buffered_iopage(XenIOS
>              break;
>          }
>          buf_req = &buf_page->buf_ioreq[rdptr % IOREQ_BUFFER_SLOT_NUM];
> -        req.size = 1UL << buf_req->size;
> -        req.count = 1;
> +        req.size = 1U << buf_req->size;
>          req.addr = buf_req->addr;
>          req.data = buf_req->data;
> -        req.state = STATE_IOREQ_READY;
>          req.dir = buf_req->dir;
> -        req.df = 1;
>          req.type = buf_req->type;
> -        req.data_is_ptr = 0;
>          xen_rmb();
>          qw = (req.size == 8);
>          if (qw) {
> @@ -1032,6 +1030,13 @@ static int handle_buffered_iopage(XenIOS
>  
>          handle_ioreq(state, &req);
>  
> +        /* Only req.data may get updated by handle_ioreq(), albeit even that
> +         * should not happen as such data would never make it to the guest.
> +         */
> +        assert(req.state == STATE_IOREQ_READY);
> +        assert(req.count == 1);
> +        assert(!req.data_is_ptr);
> +
>          atomic_add(&buf_page->read_pointer, qw + 1);
>      }
>  
> 
> 
>