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Re: [PATCH] x86: introduce "hot" and "cold" page copying functions
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jason Andryuk <jason.andryuk@xxxxxxx>
- Date: Thu, 28 Aug 2025 17:33:50 -0400
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 28 Aug 2025 21:34:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 2025-08-28 05:17, Jan Beulich wrote:
The present copy_page_sse2() is useful in case the destination page isn't
going to get touched again soon, or if we want to limit churn on the
caches. Just rename it, to fit the corresponding {clear,scrub}_page_*()
naming scheme.
For cases where latency is the most important aspect, or when it is
expected that sufficiently large parts of a destination page will get
accessed again soon after the copying, introduce a "hot" alternative.
Again use alternatives patching to select between a "legacy" and an ERMS
variant.
Don't switch any callers just yet - this will be the subject of subsequent
changes.
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
Of course both pages can be independently hot/cold, and hence more flavors
may be necessary here.
To avoid the NOP padding (also in clear_page_hot()) we could use a double
REP prefix in the replacement code (accounting for the REX one in the code
being replaced).
Did my tool chain do it automatically?
0000000000000000 <.altinstr_replacement>:
0: b9 00 10 00 00 mov $0x1000,%ecx
5: f3 f3 a4 repz rep movsb %ds:(%rsi),%es:(%rdi)
Regards,
Jason
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