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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, May 05, 2020 at 08:16:03AM +0200, 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.

I'm not sure if it would be worth to add a comment to note the
intended usage of toggle_guest_pt is to fetch data from the kernel
page tables when running in user mode. The one about using it in pairs
is certainly fine.

> While at it, add a comment ahead of toggle_guest_pt() to clarify its
> intended usage.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.