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Re: [PATCH v1-alt] xen/livepatch: Make check_for_livepatch_work() faster in the common case
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 12 Jan 2024 11:31:22 +0100
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- Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 12 Jan 2024 10:31:34 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 11.01.2024 21:11, Andrew Cooper wrote:
> When livepatching is enabled, this function is used all the time. Really do
> check the fastpath first, and annotate it likely() as this is the right answer
> 100% of the time (to many significant figures). This cuts out 3 pointer
> dereferences in the "nothing to do path".
>
> However, GCC still needs some help to persuade it not to set the full stack
> frame (6 spilled registers, 3 slots of locals) even on the fastpath.
>
> Create a new check_for_livepatch_work() with the fastpath only, and make the
> "new" do_livepatch_work() noinline. This causes the fastpath to need no stack
> frame, making it faster still.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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