Re: [PATCH v3 1/5] x86: suppress XPTI-related TLB flushes when possible

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

On Fri, May 22, 2020 at 12:00:14PM +0100, Andrew Cooper wrote:
> On 25/09/2019 16:23, Jan Beulich wrote:
> > When there's no XPTI-enabled PV domain at all, there's no need to issue
> > respective TLB flushes. Hardwire opt_xpti_* to false when !PV, and
> > record the creation of PV domains by bumping opt_xpti_* accordingly.
> >
> > As to the sticky opt_xpti_domu vs increment/decrement of opt_xpti_hwdom,
> > this is done this way to avoid
> > (a) widening the former variable,
> > (b) any risk of a missed flush, which would result in an XSA if a DomU
> >     was able to exercise it, and
> > (c) any races updating the variable.
> > Fundamentally the TLB flush done when context switching out the domain's
> > vCPU-s the last time before destroying the domain ought to be
> > sufficient, so in principle DomU handling could be made match hwdom's.
> >
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I am still concerned about the added complexity for no obvious use case.
> 
> Under what circumstances do we expect to XPTI-ness come and go on a
> system, outside of custom dev-testing scenarios?

XPTI-ness will be sticky, in the sense that once enabled cannot be
disabled anymore.

Roger.