[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [XEN PATCH 5/9] x86/cpu-policy: address violations of MISRA C Rule 10.1



On 06/10/2023 19:57, Andrew Cooper wrote:
On 06/10/2023 9:26 am, Nicola Vetrini wrote:
The COUNT_LEAVES macro is introduced to avoid using an essentially
boolean value in a subtraction.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
 xen/include/xen/lib/x86/cpu-policy.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/xen/include/xen/lib/x86/cpu-policy.h b/xen/include/xen/lib/x86/cpu-policy.h
index bab3eecda6c1..700993cc67e8 100644
--- a/xen/include/xen/lib/x86/cpu-policy.h
+++ b/xen/include/xen/lib/x86/cpu-policy.h
@@ -95,17 +95,18 @@ const char *x86_cpuid_vendor_to_str(unsigned int vendor);
 #define CPUID_GUEST_NR_EXTD       MAX(CPUID_GUEST_NR_EXTD_INTEL, \
                                       CPUID_GUEST_NR_EXTD_AMD)

+#define COUNT_LEAVES(X) ((X) - ((X) ? 1 : 0))
 /*
* Maximum number of leaves a struct cpu_policy turns into when serialised for * interaction with the toolstack. (Sum of all leaves in each union, less the
  * entries in basic which sub-unions hang off of.)
  */
-#define CPUID_MAX_SERIALISED_LEAVES                     \
-    (CPUID_GUEST_NR_BASIC +                             \
-     CPUID_GUEST_NR_FEAT   - !!CPUID_GUEST_NR_FEAT +    \
-     CPUID_GUEST_NR_CACHE  - !!CPUID_GUEST_NR_CACHE +   \
-     CPUID_GUEST_NR_TOPO   - !!CPUID_GUEST_NR_TOPO +    \
-     CPUID_GUEST_NR_XSTATE - !!CPUID_GUEST_NR_XSTATE +  \
+#define CPUID_MAX_SERIALISED_LEAVES         \
+    (CPUID_GUEST_NR_BASIC +                 \
+     COUNT_LEAVES(CPUID_GUEST_NR_FEAT) +    \
+     COUNT_LEAVES(CPUID_GUEST_NR_CACHE) +   \
+     COUNT_LEAVES(CPUID_GUEST_NR_TOPO) +    \
+     COUNT_LEAVES(CPUID_GUEST_NR_XSTATE) +  \
      CPUID_GUEST_NR_EXTD + 2 /* hv_limit and hv2_limit */ )

This may not have been a MISRA-approved calculation, but encapsulating
it like this breaks any ability to follow what's going on.

CPUID data in x86 is mostly a sparse 1-D array (BASIC, EXTD, HV blocks),
but a couple of elements in the BASIC array have arrays themselves.

The struct is laid out for O(1) access, so you can't just say
sizeof(struct) / sizeof(element).  The BASIC array has elements (0x4,
0x7, 0xb, 0xd) which hold no data because there's actually an array
elsewhere containing all the data.

So logically, it's:

(BASIC + (FEAT - 1) + (CACHE - 1) + (TOPO - 1) + (XSTATE - 1)) + EXTD + 2

And in practice I'd far rather express it with a plain -1 than a -
!!NR_, if the latter isn't an option.

Presumably MISRA would be happy with that?

If so, I can submit a patch.  There's also a typo in that the comment
that wants fixing.

~Andrew

Yes, that should be fine. I'll be happy to test that.

--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)



 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.