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SubjectRe: [RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it
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On 11.11.20 23:15, Joel Fernandes wrote:
>
> On Wed, Nov 11, 2020 at 5:13 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>>
>> On Wed, Nov 11, 2020 at 5:00 PM Alexander Graf <graf@amazon.com> wrote:
>>> On 11.11.20 22:14, Joel Fernandes wrote:
>>>>> Some hardware such as certain AMD variants don't have cross-HT MDS/L1TF
>>>>> issues. Detect this and don't enable core scheduling as it can
>>>>> needlessly slow the device done.
>>>>>
>>>>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>>>>> index dece79e4d1e9..0e6e61e49b23 100644
>>>>> --- a/arch/x86/kernel/cpu/bugs.c
>>>>> +++ b/arch/x86/kernel/cpu/bugs.c
>>>>> @@ -152,6 +152,14 @@ void __init check_bugs(void)
>>>>> #endif
>>>>> }
>>>>>
>>>>> +/*
>>>>> + * Do not need core scheduling if CPU does not have MDS/L1TF vulnerability.
>>>>> + */
>>>>> +int arch_allow_core_sched(void)
>>>>> +{
>>>>> + return boot_cpu_has_bug(X86_BUG_MDS) || boot_cpu_has_bug(X86_BUG_L1TF);
>>>
>>> Can we make this more generic and user settable, similar to the L1 cache
>>> flushing modes in KVM?
>>>
>>> I am not 100% convinced that there are no other thread sibling attacks
>>> possible without MDS and L1TF. If I'm paranoid, I want to still be able
>>> to force enable core scheduling.
>>>
>>> In addition, we are also using core scheduling as a poor man's mechanism
>>> to give customers consistent performance for virtual machine thread
>>> siblings. This is important irrespective of CPU bugs. In such a
>>> scenario, I want to force enable core scheduling.
>>
>> Ok, I can make it new kernel command line option with:
>> coresched=on
>> coresched=secure (only if HW has MDS/L1TF)
>> coresched=off
>
> Also, I would keep "secure" as the default. (And probably, we should
> modify the informational messages in sysfs to reflect this..)

I agree that "secure" should be the default. Can we also integrate into
the "mitigations" kernel command line[1] for this?


Alex

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/kernel-parameters.txt#n2839



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