On 22.11.2023 10:06, Roger Pau Monné wrote: > On Thu, Nov 16, 2023 at 02:32:47PM +0100, Jan Beulich wrote: >> This was used by VMX only, and the intended VMCS write can as well >> happen from vmx_set_tsc_offset(), invoked (directly or indirectly) >> almost immediately after the present call sites of the hook. >> vmx_set_tsc_offset() isn't invoked frequently elsewhere, so the extra >> VMCS write shouldn't raise performance concerns. >> >> Signed-off-by: Jan Beulich <jbeul...@suse.com> > > Acked-by: Roger Pau Monné <roger....@citrix.com>
Thanks. > I would be lying if I told you I understand guest TSC management in > Xen, but patch does keep the previous call sites with adding others > that are indeed not hot paths. > > I do wonder whether it would be possible to set this before the domain > is started, as AFAICT the TSC scaling ratio is only set before the > domain is started (either by domain create or other toolstack > hypercalls). > > Could we maybe further limit the write to the case where the vCPU is > not running? Possibly, but not straightforwardly: E.g. I think we'd need to actually enforce tsc_set_info() to only ever act on non-running (perhaps even not-yet-started) domains. And I mean not by looking where it's being called from at present (which may already provide such kind-of- guarantees), but in the function itself. Jan
