Messages in this thread | | | Date | Tue, 13 Feb 2018 11:24:47 -0600 | From | "Gustavo A. R. Silva" <> | Subject | Re: [PATCH] x86/microcode/intel: Use 64-bit arithmetic instead of 32-bit |
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Hi Thomas,
Quoting Thomas Gleixner <tglx@linutronix.de>:
> 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@embeddedor.com> >> --- >> 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
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