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

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Date:   Tue, 13 Feb 2018 11:24:47 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/microcode/intel: Use 64-bit arithmetic instead of
 32-bit

Hi Thomas,

Quoting Thomas Gleixner <tglx@...utronix.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@...eddedor.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