From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: "Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead
Date: Thu, 21 May 2020 17:46:27 +0100 [thread overview]
Message-ID: <d7840278-b999-65fa-40bf-2b78e5266837@citrix.com> (raw)
In-Reply-To: <24d8b606-f74b-9367-d67e-e952838c7048@suse.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
next prev parent reply other threads:[~2020-05-21 16:47 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-05 6:16 [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead Jan Beulich
2020-05-18 16:52 ` Roger Pau Monné
2020-05-21 16:46 ` Andrew Cooper [this message]
2020-05-22 10:07 ` Jan Beulich
2020-05-29 16:24 ` Andrew Cooper
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