linux-kernel - Re: [RFC][PATCH] arm64: tlb: call kvm_call_hyp once during kvm_tlb_flush_vmid

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Message-ID: <86wm0massi.wl-maz@kernel.org>
Date: Mon, 09 Feb 2026 14:35:41 +0000
From: Marc Zyngier <maz@...nel.org>
To: "yezhenyu (A)" <yezhenyu2@...wei.com>
Cc: "rananta@...gle.com" <rananta@...gle.com>,
	"will@...nel.org"
	<will@...nel.org>,
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
	"catalin.marinas@....com"
	<catalin.marinas@....com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>,
	zhengchuan <zhengchuan@...wei.com>,
	Xiexiangyou <xiexiangyou@...wei.com>,
	"guoqixin (A)" <guoqixin2@...wei.com>,
	"Mawen (Wayne)" <wayne.ma@...wei.com>
Subject: Re: [RFC][PATCH] arm64: tlb: call kvm_call_hyp once during kvm_tlb_flush_vmid_range

On Mon, 09 Feb 2026 13:14:07 +0000,
"yezhenyu (A)" <yezhenyu2@...wei.com> wrote:
> 
> From 9982be89f55bd99b3683337223284f0011ed248e Mon Sep 17 00:00:00 2001
> From: eillon <yezhenyu2@...wei.com>
> Date: Mon, 9 Feb 2026 19:48:46 +0800
> Subject: [RFC][PATCH v1] arm64: tlb: call kvm_call_hyp once during
>  kvm_tlb_flush_vmid_range
> 
> The kvm_tlb_flush_vmid_range() function is performance-critical
> during live migration, but there is a while loop when the system
> support flush tlb by range when the size is larger than MAX_TLBI_RANGE_PAGES.
>
> This results in frequent entry to kvm_call_hyp() and then a large

What is the cost of kvm_call_hyp()?

> amount of time is spent in kvm_clear_dirty_log_protect() during
> migration(more than 50%).

50% of what time? The guest's run-time? The time spent doing TLBIs
compared to the time spent in kvm_clear_dirty_log_protect()?

> So, when the address range is large than
> MAX_TLBI_RANGE_PAGES, directly call __kvm_tlb_flush_vmid to
> optimize performance.

Multiple things here:

- there is no SoB, which means that patch cannot be considered for
  merging

- there is no data showing how this change improves the situation for
  a large enough set of workloads

- there is no description of a test that could be run on multiple
  implementations to check whether this change has a positive or
  negative impact

If you want to progress this sort of things, you will need to address
these points.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.