Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jan 30, 2018 at 01:20:52AM +0000, David Dunn wrote:
> Eduardo,
> 
> This is why it would be good to have a CPUID bit that says:
> "apply SkyLake RSB stuffing."  That's preferable to "trust FMS"
> for VMware.

Agreed it would be more useful than "trust FMS".  However, I
believe a "no need to apply Skylake RSB stuffing" bit (which I
called "we promise we won't migrate to Skylake" previously) would
allow guests to enable safer behavior by default under older
hypervisors that don't support this bit.

> 
> If Intel defines such a feature flag, sets it on SkyLake, and
> Linux uses it... that would be very helpful for VMware.
> 
> I won't speak for GCE and AWS.  But hopefully they can indicate
> whether it would help them as well.

I agree that having a standard flag on the CPUID space to specify
that would be very helpful.

> 
> If Intel cannot define/implement such a flag on SkyLake, then
> maybe the engineers on this email could define a flag in the
> hypervisor specific CPUID space.  Linux would need to query
> that flag if it sees CPUID[1].ECX[31] set.  That's not as nice
> since it makes detection on bare metal and virtualization
> platforms different, but it better than keying off FMS.

Agreed.

-- 
Eduardo