Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 05/05/2020 07:16, Jan Beulich wrote:
> While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly
> expensive (but still needed only for the toggle_guest_mode() path), the
> effect of the latter on the exit-to-guest path is not insignificant.
> Move the logic into toggle_guest_mode(), on the basis that
> toggle_guest_pt() will always be invoked in pairs, yet we can't safely
> undo the setting of root_pgt_changed during the second of these
> invocations.
>
> While at it, add a comment ahead of toggle_guest_pt() to clarify its
> intended usage.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

I'm still of the opinion that the commit message wants rewriting to get
the important points across clearly.

And those are that toggle_guest_pt() is called in pairs specifically to
read kernel data structures when emulating a userspace action, and that
this doesn't modify cr3 from the guests point of view, and therefore
doesn't need the resync on exit-to-guest path.

~Andrew