Re: system locks up with CONFIG_SLS=Y; 5.17.0-rc

[Dave Hansen <dave.hansen@intel.com>]

On 3/16/22 02:51, Jamie Heilman wrote:
> I've been (somewhat unsuccessfully) trying to bisect a hard lock-up
> of my workstation that occurs when I'm running 5.17 rc kernels a few
> seconds after I start a kvm guest instance.  There is no output to
> any log, everything locks up completely, sysrq doesn't even work
> anymore.  As bisection progressed closer and closer to the branch
> where straight-line-speculation mitigation was enabled, and as bisect
> landing me between 9cdbeec40968 ("x86/entry_32: Fix segment exceptions")
> and 3411506550b1 ("x86/csum: Rewrite/optimize csum_partial()") wasn't
> resulting in clear results (my system definately starts Oopsing and
> gets so hosed up that I'm forced to reboot, but it isn't quite as dire
> as sysrq continues to function) I decided to just try a build with
> CONFIG_SLS disabled, and it turns out that works just fine.  Sooo...
> 
> This system uses a Intel Core2 Duo E8400 processor.
> working config (CONFIG_SLS=N) and dmesg at:
> http://audible.transient.net/~jamie/k/sls.config-5.17.0-rc8
> http://audible.transient.net/~jamie/k/sls.dmesg
> 
> (I don't think the dmesg of CONFIG_SLS=Y is really any different.)

If you get really ambitious, you could try to see if any of the
individual things that change based on the CONFIG_SLS #ifdef trigger
this.  Basically, turn off the config option and then go manually
enabling each of the sites.

The odd thing is that it isn't touching anything really KVM-specific.
It probably influences some KVM-specific assembly, but it's hard to see
how that might break anything.

The worrying part is:

ifdef CONFIG_SLS
  KBUILD_CFLAGS += -mharden-sls=all
endif

That's presumably a shiny, new compiler option, also known as a
relatively lightly tested compiler option.