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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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?