[MODERATED] Re: L1D-Fault KVM mitigation

[Paolo Bonzini <pbonzini@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1379 bytes --]

On 27/05/2018 20:49, speck for Linus Torvalds wrote:
> But why would you emulate halt/mwait/pause anyway? That sounds insane to 
> me. The reason you would want exit-on-halt is so that the host can do 
> something else if a vcpu goes idle, not so that it can just stay in some 
> emulated idle state.

When hlt/mwait is emulated, the thread goes to sleep.  When pause is
emulated, the thread checks if there is another CPU to yield to, but
otherwise stays running.

KVM recently grew a new mode where hlt/mwait/pause is passed directly to
the guest.  It was (partly) contributed by Amazon because that's what
they're doing in their KVM-based cloud stuff - it does all I/O in custom
hardware so all interrupts will be VT-d posted interrupts and avoid the
overhead of the vCPU thread going to sleep and back running.

There is a problem though.  The only way to know if the guest is in
hlt/mwait/pause, is to cause a vmexit, e.g. with an IPI, and read the VM
control state (which you can only do from the CPU that was running it!).
 So even for the "idle in the guest" case you pretty much have to do
synchronization.

Paolo

> If you want to go to low-power mode, you'd just let the halt/mwait happen 
> inside the guest.
> 
> But to be honest, I haven't actually checked what kvm users (or xen, or 
> whatever) really do. Am I missing something?