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

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Message-ID: <d3c65087-c846-439c-8f46-ceb899809b70@intel.com>
Date: Tue, 22 Jul 2025 14:33:30 -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 2:16 PM, Reinette Chatre wrote:
> 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?
> 

One more thing, could you please check the kernel logs for any other issues
when that VM boots up?
I would like to check if it contains a warning with text like:
"x86 CPU feature dependency check failure"

Reinette