[Xen-API] [PATCH 4 of 5 v4] tools/ocaml: libxc bindings: Fix stub_xc_readconsolering()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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 v3:
 * Use caml_alloc_string/memcpy in preference to caml_copy_string to allow
   for the possibility of NULL characters in the middle of the conring
Changes since v2:
 * Tweak style and remove redundant variables
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 c7b82dfbec34 -r 5bd08f35b362 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 +523,47 @@ 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;
+       static uint64_t conring_size = 0;
+       unsigned int nr_chars;
+       DECLARE_HYPERCALL_BUFFER(char, ring);
        int retval;
 
        CAMLparam1(xch);
+       CAMLlocal1(conring);
+
+       if (!conring_size)
+       {
+               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;
+       }
+
+       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);
+       retval = xc_readconsolering_buffer(_H(xch), HYPERCALL_BUFFER(ring),
+                                          &nr_chars, 0, 0, NULL);
        caml_leave_blocking_section();
 
        if (retval)
+       {
+               xc_hypercall_buffer_free(_H(xch), ring);
                failwith_xc(_H(xch));
+       }
 
-       ring[size] = '\0';
-       CAMLreturn(caml_copy_string(ring));
+       conring = caml_alloc_string(nr_chars);
+       memcpy(String_val(conring), ring, nr_chars);
+       xc_hypercall_buffer_free(_H(xch), ring);
+       CAMLreturn(conring);
 }
 
 CAMLprim value stub_xc_send_debug_keys(value xch, value keys)

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