Re: [Xen-devel] [PATCH v3] x86/time: correctly honor late clearing of TSC related feature flags

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/02/17 08:39, Jan Beulich wrote:
> As such clearing of flags may have an impact on the selected rendezvous
> function, defer the establishing of a rendezvous function other than
> the initial default one (std) until after all APs have been brought up.
>
> But don't allow such feature flags to be cleared during CPU hotplug:
> Platform and local system times may have diverged significantly by
> then, potentially causing noticeably (even if only temporary) strange
> behavior. As we're anyway expecting only sufficiently similar CPUs to
> appear during hotplug, this shouldn't be introducing new limitations.
>
> Reported-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
> v3: Drop original approach entirely - defer everything to
>     verify_tsc_reliability(), making for quite a bit smaller a patch.
>
> Note: Considering that tsc_check_writability() checks
>       TSC_RELIABLE, it being run before that feature flag has obtained
>       its final value seems problematic too. Should we defer that call
>       too?

Do we know which CPUs this applies to?  Might we be safe by being only
64bit these days?

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel