Re: [Xen-devel] [PATCH 4 of 5 v2] tools/ocaml: libxc bindings: Fix stub_xc_readconsolering()

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-02-21 at 15:46 +0000, Andrew Cooper wrote:
> There are two problems with this function:
>   * The caml_{enter,leave}_blocking_section() calls mean that two threads can
>     compete for use of the static ring[] buffer.
>   * It fails to retrieve the entire buffer if the hypervisor has used 32K or
>     more of its console ring.
> 
> Rewrite the function using the new xc_consoleringsize() and
> xc_readconsolering_buffer() functions to efficiently grab the entire console
> ring.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> --
> Changes since v1:
>  * Convert conring_size to being static to avoid needless hypercalls
>  * Fix memory due to ordering of failwith_xc() and xc_hypercall_buffer_free()
>  * Remove useless CAMLreturns()
> 
> diff -r 1e6c7f7cec6f -r 1774a72fde4a tools/ocaml/libs/xc/xenctrl_stubs.c
> --- a/tools/ocaml/libs/xc/xenctrl_stubs.c
> +++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
> @@ -523,27 +523,50 @@ CAMLprim value stub_xc_evtchn_reset(valu
>       CAMLreturn(Val_unit);
>  }
>  
> -
> -#define RING_SIZE 32768
> -static char ring[RING_SIZE];
> -
>  CAMLprim value stub_xc_readconsolering(value xch)
>  {
> -     unsigned int size = RING_SIZE - 1;
> -     char *ring_ptr = ring;
> -     int retval;
> +     static int have_conring_size = 0;
> +     static uint64_t conring_size;

Checking if conring_size == 0 (or ~0 if you prefer) is functionally
equivalent to the additional variable I think.

> +
> +     unsigned int nr_chars;
> +     DECLARE_HYPERCALL_BUFFER(char, ring);
> +     int r;
>  
>       CAMLparam1(xch);
> +     CAMLlocal1(conring);
> +
> +     if ( ! have_conring_size )

This overuse of spaces is neither Xen style nor the prevailing style in
this file AFAICT. The prevailing style appears to be Linux-ish, I
suggest to stick with that.

> +     {
> +             if ( xc_consoleringsize(_H(xch), &conring_size) )
> +                     failwith_xc(_H(xch));
> +
> +             /* Round conring_size up to the next page, for NULL terminator
> +                and slop from a race with printk() in the hypervisor. */
> +             conring_size = (conring_size + XC_PAGE_SIZE) & XC_PAGE_MASK;
> +             have_conring_size = 0;

If you do want to do things this way then surely this has to be = 1 ?

> +     }
> +
> +     nr_chars = conring_size-1;
> +     ring = xc_hypercall_buffer_alloc(_H(xch), ring, conring_size);
> +
> +     if ( ! ring )
> +             caml_raise_out_of_memory();
>  
>       caml_enter_blocking_section();
> -     retval = xc_readconsolering(_H(xch), ring_ptr, &size, 0, 0, NULL);
> +     r = xc_readconsolering_buffer(_H(xch), HYPERCALL_BUFFER(ring),
> +                                   &nr_chars, 0, 0, NULL);
>       caml_leave_blocking_section();
>  
> -     if (retval)
> +     if ( r )

The s/retval/r/ seems a bit unneeded.

> +     {
> +             xc_hypercall_buffer_free(_H(xch), ring);
>               failwith_xc(_H(xch));
> +     }
>  
> -     ring[size] = '\0';
> -     CAMLreturn(caml_copy_string(ring));
> +     ring[nr_chars] = '\0';
> +     conring = caml_copy_string(ring);
> +     xc_hypercall_buffer_free(_H(xch), ring);
> +     CAMLreturn(conring);
>  }
>  
>  CAMLprim value stub_xc_send_debug_keys(value xch, value keys)



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