linux-kernel - Re: [PATCH] x86/resctrl: avoid divide by 0 num

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Message-ID: <2249977a-e291-494f-8c84-f16ec9fba10b@intel.com>
Date: Tue, 22 Jul 2025 14:16:35 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Barret Rhoden <brho@...gle.com>
CC: Tony Luck <tony.luck@...el.com>, Dave Martin <Dave.Martin@....com>, "James
 Morse" <james.morse@....com>, <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] x86/resctrl: avoid divide by 0 num_rmid

Hi Barret,

On 7/22/25 11:51 AM, Barret Rhoden wrote:
> On 7/22/25 2:19 PM, Reinette Chatre wrote:
>> Hi Barret,
>>
>> On 7/21/25 11:00 AM, Barret Rhoden wrote:
>>> x86_cache_max_rmid's default is -1.  If the hardware or VM doesn't set
>>> the right cpuid bits, num_rmid can be 0.
>>>
>>> Signed-off-by: Barret Rhoden <brho@...gle.com>
>>>
>>> ---
>>> I ran into this on a VM on granite rapids.  I guess the VMM told the
>>> kernel it was a GNR, but didn't set all the cache/rsctl bits.
>>>
>>
>> The -1 default of x86_cache_max_rmid is assigned if the hardware does not
>> support *any* L3 monitoring. Specifically:
>>
>> resctrl_cpu_detect():
>>     if (!cpu_has(c, X86_FEATURE_CQM_LLC)) {
>>         c->x86_cache_max_rmid  = -1;
>>         ...
>>     }
>>
>> The function modified by this patch, rdt_get_mon_l3_config() only runs if
>> the hardware supports one or more of the L3 monitoring sub-features
>> (X86_FEATURE_CQM_OCCUP_LLC, X86_FEATURE_CQM_MBM_TOTAL, or
>> X86_FEATURE_CQM_MBM_LOCAL) that depend on X86_FEATURE_CQM_LLC per cpuid_deps[].
>>
>> I tried to reproduce the issue on real hardware by using clearcpuid to
>> disable X86_FEATURE_CQM_LLC and the CPUID dependencies did the right thing
>> by automatically disabling X86_FEATURE_CQM_OCCUP_LLC, X86_FEATURE_CQM_MBM_TOTAL,
>> X86_FEATURE_CQM_MBM_LOCAL, not running rdt_get_mon_l3_config() at all, and
>> not even attempt to enumerate any of the L3 monitoring details.
>>
>> What are the symptoms when you encounter this issue?
> 
> Linux crashes during boot with a divide error, and the splat backtrace is in rdt_get_mon_l3_config().
> 
>> Would it be possible to send me the CPUID flags of leaf 7, subleaf 0 as
>> well as all sub-leaves of leaf 0xF?
> 
> # ./cpuid 0x7 0
> CPUID for Leaf 0x00000007, Sublevel 0x00000000:
>     eax: 00000002
>     ebx: f1bf2ffb

Looks like X86_FEATURE_CQM_LLC is off.

>     ecx: 1b415f7e
>     edx: bc814410
> 
> # ./cpuid 0x7 1
> CPUID for Leaf 0x00000007, Sublevel 0x00000001:
>     eax: 00201c30
>     ebx: 00000000
>     ecx: 00000000
>     edx: 00084000
> 
> # ./cpuid 0x7 2
> CPUID for Leaf 0x00000007, Sublevel 0x00000002:
>     eax: 00000000
>     ebx: 00000000
>     ecx: 00000000
>     edx: 0000003f
> 

Thank you. Could you please also pass on all sub-leaves of leaf 0xF?

Thank you very much.

Reinette