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

[Joel Fernandes]

On Wed, Nov 11, 2020 at 5:00 PM Alexander Graf <graf@xxxxxxxxxx> wrote:
>
>
>
> On 11.11.20 22:14, Joel Fernandes wrote:
> >
> > Sorry, forgot to CC +Tom Lendacky . Please take a look Tom, thanks.
> >
> > On Wed, Nov 11, 2020 at 4:10 PM Joel Fernandes (Google)
> > <joel@xxxxxxxxxxxxxxxxx> 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.
> >>
> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> >> ---
> >>   arch/x86/kernel/cpu/bugs.c | 8 ++++++++
> >>   kernel/sched/core.c        | 7 +++++++
> >>   kernel/sched/sched.h       | 5 +++++
> >>   3 files changed, 20 insertions(+)
> >>
> >> 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

Does that work for you?  Others, thoughts?

Thanks,
- Joel