[MODERATED] Re: [PATCH 2/3] v4 more sampling fun 2

["Luck, Tony" <tony.luck@intel.com>]

On Fri, Mar 27, 2020 at 09:20:41AM -0700, speck for mark gross wrote:
> On Wed, Mar 25, 2020 at 10:19:23PM -0500, speck for Josh Poimboeuf wrote:
> > > +static void __init srbds_select_mitigation(void)
> > > +{
> > > +	u64 ia32_cap;
> > > +
> > > +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
> > > +		srbds_mitigation = SRBDS_HYPERVISOR;
> > > +		return;
> > > +	}
> > 
> > It's confusing that this comes before the X86_BUG_SRBDS check.  Is that
> > on purpose?
> VM vendors on keybase wanted it so.

No. They don't want this. It results in all multi-socket servers
(which are not affected by SRBDS) saying the mitigation status is
unknown.

The preferred order to check is:

1) Am I on the affected list?

2) Am I running under a hypervisor?


Initially Andrew Cooper had asked for the hypervisor check to come
first. But he has since relented.  Below is a comment to explain
why it is practially better to check for prescence on the affected
list *before* checking for hypervisor:

/*
 * This test relies on the CPUID values of vendor, family, model,
 * stepping which might not reflect the real hardware when we are
 * running as a guest. But VMM vendors have asked that we do this
 * before the X86_FEATURE_HYPERVISOR test since this provides better
 * guidance to users in most real situations.
 */

-Tony