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Re: [PATCH] x86/microcode/intel: Use 64-bit arithmetic instead of 32-bit




Hi Thomas,

Quoting Thomas Gleixner <tglx@xxxxxxxxxxxxx>:

On Tue, 13 Feb 2018, Gustavo A. R. Silva wrote:

Add suffix ULL to constant 1024 in order to give the compiler complete
information about the proper arithmetic to use. Notice that this
constant is used in a context that expects an expression of type
u64 (64 bits, unsigned).

The expression c->x86_cache_size * 1024 is currently being evaluated
using 32-bit arithmetic.

Addresses-Coverity-ID: 1464429
Signed-off-by: Gustavo A. R. Silva <garsilva@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/microcode/intel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index f7c55b0..e5edb92 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -982,7 +982,7 @@ static struct microcode_ops microcode_intel_ops = {

 static int __init calc_llc_size_per_core(struct cpuinfo_x86 *c)
 {
-	u64 llc_size = c->x86_cache_size * 1024;
+	u64 llc_size = c->x86_cache_size * 1024ULL;

x86_cache_size is 'int', so you really want to cast c->x86_cache_size to
(u64) for correctness sake.

Aside of that the patch is really purely cosmetic at the moment because the
largest LLC sizes are still below the 3 digit MB range which fits into
32bit quite well. You'd need to have a CPU with >= 2G LLC to create a
problem.

But looking at c->x86_cache_size again. It's int because it's set to -1
initially which is then changed if CPUid or general CPU info gives real
information about the cache size. The only place where that matters is the
/proc/cpuinfo output:

	if (c->x86_cache_size >= 0)
		seq_printf(m, "cache size\t: %d KB\n", c->x86_cache_size);

which is silly, because that really can be done with:

	if (c->x86_cache_size)

as there is no point in printing 'cache size 0KB', which means
x86_cache_size can be made unsigned int, which makes sense because cache
size < 0 does not at all.

So instead of doing this purely mechanical cosmetic change to make a static
checker shut up, I'd like to see a proper cleanup of that thing.


Yeah, actually I was curious about why x86_cache_size is signed instead of unsigned. You've made it clear now.

I will change it to be of type unsigned int and make the proper changes to the rest of code in which x86_cache_size is being used.

Also, I'm curious about the types of the rest of the related variables:

        /* Cache QoS architectural values: */
        int                     x86_cache_max_rmid;     /* max index */
        int                     x86_cache_occ_scale;    /* scale to bytes */
        int                     x86_power;


Maybe they need some cleanup too.

Thanks for the feedback.
--
Gustavo