Re: [RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it

[Alexander Graf]

On 11.11.20 23:15, Joel Fernandes wrote:
> On Wed, Nov 11, 2020 at 5:13 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Nov 11, 2020 at 5:00 PM Alexander Graf <graf@xxxxxxxxxx> 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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