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Re: [PATCH 5/5] x86/ioapic: Drop function pointers from __ioapic_{read,write}_entry()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 12 Nov 2021 11:43:53 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 12 Nov 2021 10:44:16 +0000
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On 11.11.2021 18:57, Andrew Cooper wrote:
> Function pointers are expensive, and the raw parameter is a constant from all
> callers, meaning that it predicts very well with local branch history.
The code change is fine, but I'm having trouble with "all" here: Both
functions aren't even static, so while callers in io_apic.c may
benefit (perhaps with the exception of ioapic_{read,write}_entry(),
depending on whether the compiler views inlining them as warranted),
I'm in no way convinced this extends to the callers in VT-d code.
Further ISTR clang being quite a bit less aggressive about inlining,
so the effects might not be quite as good there even for the call
sites in io_apic.c.
Can you clarify this for me please?
Jan
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