Intended behavior/usage of SSBD setting

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Hello,

As part of some follow up improvements to my VIRT_SPEC_CTRL series we
have been discussing what the usage of SSBD should be for the
hypervisor itself.  There's currently a `spec-ctrl=ssbd` option [0],
that has an out of date description, as now SSBD is always offered to
guests on AMD hardware, either using SPEC_CTRL or VIRT_SPEC_CTRL.

It has been pointed out by Andrew that toggling SSBD on AMD using
VIRT_SPEC_CTRL or the non-architectural way (MSR_AMD64_LS_CFG) can
have a high impact on performance, and hence switching it on every
guest <-> hypervisor context switch is likely a very high
performance penalty.

It's been suggested that it could be more appropriate to run Xen with
the guest SSBD selection on those systems, however that clashes with
the current intent of the `spec-ctrl=ssbd` option.

I hope I have captured the expressed opinions correctly in the text
above.

I see two ways to solve this:

 * Keep the current logic for switching SSBD on guest <-> hypervisor
   context switch, but only use it if `spec-ctrl=ssbd` is set on the
   command line.

 * Remove the logic for switching SSBD on guest <-> hypervisor context
   switch, ignore setting of `spec-ctrl=ssbd` on those systems and run
   hypervisor code with the guest selection of SSBD.

Which has raised me the question of whether there's an use case
for always running hypervisor code with SSBD enabled, or that's no
longer relevant if we always offer guests a way for them to toggle the
setting when required.

I would like to settle on a way forward, so we can get this fixed
before 4.17.

Thanks, Roger.

[0] 
https://xenbits.xen.org/docs/unstable/misc/xen-command-line.html#spec-ctrl-x86