linux-kernel - Re: [PATCH v2 0/5] Support the FEAT

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Message-ID: <90180222-5399-442c-b7a3-e65b7d0e1378@huawei.com>
Date: Fri, 21 Nov 2025 18:21:16 +0800
From: z00939249 <zhengtian10@...wei.com>
To: Marc Zyngier <maz@...nel.org>, Tian Zheng <zhengtian10@...wei.com>
CC: <oliver.upton@...ux.dev>, <catalin.marinas@....com>, <corbet@....net>,
	<pbonzini@...hat.com>, <will@...nel.org>, <linux-kernel@...r.kernel.org>,
	<yuzenghui@...wei.com>, <wangzhou1@...ilicon.com>, <yezhenyu2@...wei.com>,
	<xiexiangyou@...wei.com>, <zhengchuan@...wei.com>, <joey.gouly@....com>,
	<kvmarm@...ts.linux.dev>, <kvm@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-doc@...r.kernel.org>,
	<suzuki.poulose@....com>
Subject: Re: [PATCH v2 0/5] Support the FEAT_HDBSS introduced in Armv9.5

On 2025/11/21 17:54, Marc Zyngier wrote:
> On Fri, 21 Nov 2025 09:23:37 +0000,
> Tian Zheng <zhengtian10@...wei.com> wrote:
>>
>> This series of patches add support to the Hardware Dirty state tracking
>> Structure(HDBSS) feature, which is introduced by the ARM architecture
>> in the DDI0601(ID121123) version.
>>
>> The HDBSS feature is an extension to the architecture that enhances
>> tracking translation table descriptors' dirty state, identified as
>> FEAT_HDBSS. The goal of this feature is to reduce the cost of surveying
>> for dirtied granules, with minimal effect on recording when a granule
>> has been dirtied.
>>
>> The purpose of this feature is to make the execution overhead of live
>> migration lower to both the guest and the host, compared to existing
>> approaches (write-protect or search stage 2 tables).
>>
>> After these patches, users(such as qemu) can use the
>> KVM_CAP_ARM_HW_DIRTY_STATE_TRACK ioctl to enable or disable the HDBSS
>> feature before and after the live migration.
>>
>> This feature is similar to Intel's Page Modification Logging (PML),
>> offering hardware-assisted dirty tracking to reduce live migration
>> overhead. With PML support expanding beyond Intel, HDBSS introduces a
>> comparable mechanism for ARM.
> 
> Where is the change log describing what was changed compared to the
> previous version?
> 
> We gave you extensive comments back in March. You never replied to the
> feedback. And you now dump a whole set of patches, 6 months later,
> without the slightest indication of what has changed?
> 
> Why should we make the effort to review this again?

Apologies for the lack of proper changelog and the delayed follow-up on 
the feedback provided in March. This was an oversight on our part during 
the transition of maintainership for the HDBSS patch series. We 
sincerely appreciate the thorough comments you shared earlier and regret 
not responding in a timely manner.

Below is a summary of the changes made from v1 to v2.

v1:
https://lore.kernel.org/kvm/20250311040321.1460-1-yezhenyu2@huawei.com/

v1->v2 changes:
- Removed redundant macro definitions and switched to tool-generated.
- Split HDBSS interface and implementation into separate patches.
- Integrate system_supports_hdbss() into ARM feature initialization.
- Refactored HDBSS data structure to store meaningful values instead
of raw register contents.
- Fixed permission checks when applying DBM bits in page tables to
prevent potential memory corruption.
- Removed unnecessary dsb instructions.
- Drop the debugging printks.
- Merged the two patches "using ioctl to enable/disable the HDBSS
feature" and "support to handle the HDBSSF event" into one.

We apologize again for the delay and the missing changelog, and we 
greatly appreciate your time in reviewing this updated version.