On 05/24/2018 08:38 AM, speck for Jiri Kosina wrote:
>> That means HT off, but it's still better than having absolutely nothing.
> Will we actually be enforcing switching SMT off (before anything better 
> exists) by either offlining all the siblings or forcing them to idle at 
> the moment first virtual machine gets started, from the kernel directly?
> 
> This seems like this policy would better be enforced by userspace 
> (libvirt?), but kernel should probably at least warn on affected CPUs if 
> it detects this is being violated.

The most straightforward thing is to do trigger the same behavior as
"noht" as part of our arch/x86/kernel/cpu/bugs.c code whenever KVM is
compile-time enabled.

I think we have to do that by default, but allow folks to override it if
they want, like if they know KVM will never get used.