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From: Marc Zyngier <maz@kernel.org>
To: zhukeqian <zhukeqian1@huawei.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH v2 0/8] KVM: arm64: Support HW dirty log based on DBM
Date: Tue, 28 Jul 2020 08:52:22 +0100
Message-ID: <2a81b2a0e1bd1e7523760a8074b9ec6e@kernel.org> (raw)
In-Reply-To: <884bd32d-c8ba-6237-82f6-1769a83289f5@huawei.com>

On 2020-07-28 03:11, zhukeqian wrote:
> Hi Marc,

[...]

>> But you are still reading the leaf entries of the PTs, hence defeating
>> any sort of prefetch that the CPU could do for you. And my claim is
>> that reading the bitmap is much faster than parsing the PTs. Are you
>> saying that this isn't the case?
> I am confused here. MMU DBM just updates the S2AP[1] of PTs, so dirty
> information
> is not continuous. The smallest granularity of read instruction is one
> byte, we must
> read one byte of each PTE to determine whether it is dirty. So I think
> the smallest
> reading amount is 512 bytes per 2MB.

Which is why using DBM as a way to implement dirty-logging doesn't work.
Forcing the vcpu to take faults in order to update the dirty bitmap
has the benefit of (a) telling you exactly what page has been written 
to,
(b) *slowing the vcpu down*.

See? no additional read, better convergence ratio because you are not
trying to catch up with a vcpu running wild. You are in control of the
dirtying rate, not the vcpu, and the information you get requires no
extra work (just set the corresponding bit in the dirty bitmap).

Honestly, I think you are looking at the problem the wrong way.
DBM at S2 is not a solution to anything, because the information is
way too sparse, and  it doesn't solve the real problem, which is
the tracking of dirty pages caused by devices.

As I said twice before, I will not consider these patches without
a solution to the DMA problem, and I don't think DBM is part of
that solution.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...
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Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-02 13:55 Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 1/8] KVM: arm64: Set DBM bit for writable PTEs Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 2/8] KVM: arm64: Scan PTEs to sync dirty log Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 3/8] KVM: arm64: Modify stage2 young mechanism to support hw DBM Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 4/8] KVM: arm64: Save stage2 PTE dirty status if it is covered Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 5/8] KVM: arm64: Steply write protect page table by mask bit Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 6/8] KVM: arm64: Add KVM_CAP_ARM_HW_DIRTY_LOG capability Keqian Zhu
2020-07-06  1:08   ` zhukeqian
2020-07-02 13:55 ` [PATCH v2 7/8] KVM: arm64: Sync dirty log parallel Keqian Zhu
2020-07-02 13:55 ` [PATCH v2 8/8] KVM: Omit dirty log sync in log clear if initially all set Keqian Zhu
2020-07-06  1:28 ` [PATCH v2 0/8] KVM: arm64: Support HW dirty log based on DBM zhukeqian
2020-07-06  7:54   ` Marc Zyngier
2020-07-13  2:47     ` zhukeqian
2020-07-13 14:53       ` Marc Zyngier
2020-07-28  2:11         ` zhukeqian
2020-07-28  7:52           ` Marc Zyngier [this message]
2020-07-28  8:32             ` zhukeqian

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