Re: [RFC][PATCH] arm64: tlb: call kvm_call_hyp once during kvm_tlb_flush_vmid_range

[Marc Zyngier <maz@kernel.org>]

On Thu, 12 Feb 2026 12:02:33 +0000,
"yezhenyu (A)" <yezhenyu2@huawei.com> wrote:
> 
> Thanks for your review.
> 
> On 2026/2/9 22:35, Marc Zyngier wrote:
> > On Mon, 09 Feb 2026 13:14:07 +0000,
> > "yezhenyu (A)" <yezhenyu2@huawei.com> wrote:
> >> 
> >>  From 9982be89f55bd99b3683337223284f0011ed248e Mon Sep 17 00:00:00 2001
> >> From: eillon <yezhenyu2@huawei.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()?
> > 
> 
> Most cost of kvm_tlb_flush_vmid_range() is __tlb_switch_to_host(), which
> is called in every __kvm_tlb_flush_vmid/__kvm_tlb_flush_vmid_range.

That was not my question: you indicate that frequent calls to
kvm_call_hyp() are making things costly. I find this assertion
surprising, given that on a VHE system, this is exactly *nothing*.

> 
> >> 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()?
> > 
> 
> kvm_clear_dirty_log_protect() cost more than 50% time during
> ram_find_and_save_block(), but not every time.
> I captured the flame graph during the live migration, and the
> distribution of several key functions is as follows(sorry I
> cannot transfer the SVG files outside my company):
> 
>     ram_find_and_save_block(): 84.01%
>         memory_region_clear_dirty_bitmap(): 33.40%
>             kvm_clear_dirty_log_protect(): 26.74%
>                 kvm_arch_flush_remote_tlbs_range(): 9.67%
>                     __tlb_switch_to_host(): 9.51%
>                 kvm_arch_mmu_enable_log_dirty_pt_masked(): 9.38%
>         ram_save_target_page_legacy(): 43.41%
> 
> The memory_region_clear_dirty_bitmap() cost about 40% of
> ram_find_and_save_block(), and the kvm_arch_flush_remote_tlbs_range()
> cost about 29% of memory_region_clear_dirty_bitmap().
> 
> And after the patch apply, the distribution of several key functions is
> as follows:
> 
>     ram_find_and_save_block(): 53.84%
>         memory_region_clear_dirty_bitmap(): 2.28%
>             kvm_clear_dirty_log_protect(): 1.75%
>                 kvm_arch_flush_remote_tlbs_range(): 0.03%
>                     __tlb_switch_to_host(): 0.03%
>                 kvm_arch_mmu_enable_log_dirty_pt_masked(): 0.96%
>         ram_save_target_page_legacy(): 38.97%
>
>
> The memory_region_clear_dirty_bitmap() cost about 4% of
> ram_find_and_save_block(), and the kvm_arch_flush_remote_tlbs_range()
> cost about 1% of memory_region_clear_dirty_bitmap().

What is ram_find_and_save_block()? userspace code?

> 
> >> 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
> > 
> If there are no other issues with this patch, I can resend it with the
> SoB (Signed-off-by) tag.
> 
> 
> > - 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
> 
> This patch affected the migration bandwidth during the live migration.
> With the same physical bandwidth, the optimization effect of this patch
> can be observed by monitoring the real live migration bandwidth.
> 
> I have test this in an RDMA-like environment, the physical bandwidth is
> about 100GBps; without this patch, the migration bandwidth is below 10
> GBps, and after this patch apply, the migration bandwidth can reach 50
> GBps.

Again: how can other people reproduce your findings? Please provide a
test, and its exact configuration. If this truly results in a 5x
improvement, it shouldn't be hard to reproduce.

Thanks,

	M.

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