Messages in this thread | | | Subject | Re: [RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it | From | Alexander Graf <> | Date | Thu, 12 Nov 2020 21:01:47 +0100 |
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On 12.11.20 16:28, Joel Fernandes wrote: > > On Thu, Nov 12, 2020 at 03:52:32PM +0100, Alexander Graf wrote: >> >> >> On 12.11.20 14:40, Joel Fernandes wrote: >>> >>> On Wed, Nov 11, 2020 at 11:29:37PM +0100, Alexander Graf wrote: >>>> >>>> >>>> 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. >>> >>> Ok. >>> >>>> Can we also integrate into the "mitigations" kernel command line[1] for this? >>> >>> Sure, the integration into [1] sounds conceptually fine to me however it is >>> not super straight forward. Like: What if user wants to force-enable >>> core-scheduling for the usecase you mention, but still wants the cross-HT >>> mitigation because they are only tagging VMs (as in your usecase) and not >>> other tasks. Idk. >> >> Can we roll this backwards from what you would expect as a user? How about >> we make this 2-dimensional? >> >> coresched=[on|off|secure][,force] >> >> where "on" means "core scheduling can be done if colors are set", "off" >> means "no core scheduling is done" and "secure" means "core scheduling can >> be done on MDS or L1TF if colors are set". > > So support for this force thing is not there ATM in the patchset. We can > always incrementally add it later. I personally don't expect users to be Ok > with tagging every single task as it is equivalent to disabling SMT and makes > coresched useless.
It just flips the default from "always consider everything safe" to "always consider everything unsafe". Inside a cgroup, you can still set the same color to make use of siblings.
Either way, I agree that it can be a follow-up.
> >> The "force" option would then mean "apply a color to every new task". >> >> What then happens with mitigations= is easy. "auto" means >> "coresched=secure". "off" means "coresched=off" and if you want to force >> core scheduling for everything if necessary, you just do mitigations=auto >> coresched=auto,force. >> >> Am I missing something obvious? :) > > I guess I am confused for the following usage: > mitigations=auto,nosmt coresched=secure > > Note that auto,nosmt disables SMT selectively *only if needed*. Now, you add > coresched=secure to the mix. Should auto,nosmt disable SMT or not? It should be > disabled if the user did not tag anything (because system is insecure). It > should be enabled, if they tagged things. So it really depends on user doing > the right thing. And it is super confusing already -- I would just rather > keep coresched= separate from mitigations= and document it properly. TBH- > coresched does require system admin / designer to tag things as needed so why > pretend that its easy to configure anyway? :)
coresched=secure still won't allow you to trust your system without thinking about it, while nosmt does. So I would say that nosmt does not imply anything for coresched (until ,force is available, then we're talking ...)
The main thing I'm interested in though is mitigations=off. When you know you only care about performance and not side channel security (HPC for example), then you can in general just set mitigations=off. That should definitely affect the core scheduling setting as well.
Alex
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