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

[Jan Beulich <jbeulich@suse.com>]

On 21.05.2020 18:46, Andrew Cooper wrote:
> 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.

Is this

"toggle_guest_pt() is called in pairs, to read guest kernel data
 structures when emulating a guest userspace action. Hence this doesn't
 modify cr3 from the guest's point of view, and therefore doesn't need
 any resync on the exit-to-guest path. Therefore move the updating of
 ->pv_cr3 and ->root_pgt_changed into toggle_guest_mode(), since undoing
 the changes during the second of these invocations wouldn't be a safe
 thing to do."

any better?

Jan