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Re: [PATCH v3] x86/vmx: Rewrite vmx_sync_pir_to_irr() to be more efficient
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 5 Mar 2025 12:36:48 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 05 Mar 2025 11:37:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05.03.2025 00:22, Andrew Cooper wrote:
> There are two issues. First, pi_test_and_clear_on() pulls the cache-line to
> the CPU and dirties it even if there's nothing outstanding, but the final
> bitmap_for_each() is O(256) when O(8) would do, and would avoid multiple
> atomic updates to the same IRR word.
>
> Rewrite it from scratch, explaining what's going on at each step.
>
> Bloat-o-meter reports 177 -> 145 (net -32), but real improvement is the
> removal of calls to __find_{first,next}_bit() hidden behind bitmap_for_each().
Nit: As said in reply to v2, there are no underscores on the two find
functions bitmap_for_each() uses.
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>
> https://gitlab.com/xen-project/people/andyhhp/xen/-/pipelines/1699791518
>
> v3:
> * Fix IRR scatter address calculation
> * Spelling/Grammar improvements
The description starting with "There are two issues" I fear it still
doesn't become quite clear what the 2nd issue is. I can only assume it's
the use of bitmap_for_each() that now goes away.
Preferably with this tweaked a little further
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Jan
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