Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

[Andrea Arcangeli]

On Wed, Jan 10, 2018 at 06:59:54AM -0800, Dave Hansen wrote:
> On 01/10/2018 06:10 AM, Andrea Arcangeli wrote:
> > Tim and Dave please comment too, Tim you originally wrote that code
> > that leaves IBRS always on and never toggles it in the kernel entry
> > point so you must know full well if Arjan is correct that you must
> > toggle IBRS every time you enter kernel and in turn ibrs_enabled 2
> > isn't valid mode.
> 
> Hi Andrea,
> 
> The "writing IBRS=1 acts as a barrier when it is already IBRS=1"
> behavior is something which I misunderstood in the past.  Thanks, Arjan,
> for clearing it up.

"writing IBRS=1 acts as a barrier when it is already IBRS=1" would
have been much clearer wording frankly. IBPB is IBP "Barrier", but
also IBRS is a barrier, no problem :).

So we'll add a dummy IBRS write to SPEC_CTRL in kernel entry and
vmexit so that it is compliant with all released microcodes that may
require it, also when ibrs_enabled is 2. Can you confirm?

Can you also tell if IBRS must be written as a barrier to SPEC_CTRL in
return to userland (kernel exit) when ibrs_enabled 2? Generally we
wouldn't run a barrier there with ibrs_enabled 2, but absolutely
nothing is intuitive here so I need to ask explicitly.