Re: [Xen-devel] [PATCH v3] x86/time: Don't use virtual TSC if host and guest frequencies are equal

[Olaf Hering <olaf@xxxxxxxxx>]

Am Fri, 17 Mar 2017 14:15:26 -0400
schrieb Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>:

> Commit 82713ec8d2 ("x86: use native RDTSC(P) execution when guest and
> host frequencies are the same") left out optimization for PV guests
> when host and guest run at the same frequency.
> 
> For such a case we should be able not to use virtual TSC regardless
> of whether we are runing before or after a migration (i.e. regardless
> of incarnation value).


I think this part of tscmode.txt may needs an update now.
Doesn't the "always emulated for PV" meant the part which was changed?


...
The default mode (tsc_mode==0) checks TSC-safeness of the underlying
hardware on which the virtual machine is launched.  If it is
TSC-safe, rdtsc will execute at hardware speed; if it is not, rdtsc
will be emulated.  Once a virtual machine is save/restored or migrated,
however, there are two possibilities:  For a paravirtualized (PV) domain,
TSC will always be emulated.  For a fully-virtualized (HVM) domain,
TSC remains native IF the source physical machine and target physical machine
have the same TSC frequency; else TSC is emulated.  Note that, though
emulated, the "apparent" TSC frequency will be the TSC frequency
of the initial physical machine, even after migration.
...

Olaf

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