From: Gabriel Krisman Bertazi <krisman@suse.de>
To: catalin.marinas@arm.com
Cc: will@kernel.org, linux-arm-kernel@lists.infradead.org,
broonie@kernel.org
Subject: Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path
Date: Mon, 16 Jan 2023 17:41:35 -0300 [thread overview]
Message-ID: <871qnuciv4.fsf@suse.de> (raw)
In-Reply-To: <20230109151955.8292-1-krisman@suse.de> (Gabriel Krisman Bertazi's message of "Mon, 9 Jan 2023 12:19:55 -0300")
Gabriel Krisman Bertazi <krisman@suse.de> writes:
> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> in the hypervisor. In fact, ARM documentation mentions some feature
> registers are not supposed to be accessed frequently by the OS, and
> therefore should be emulated for guests [1].
>
> Commit 0388f9c74330 ("arm64: mm: Implement
> arch_wants_old_prefaulted_pte()") introduced a read of this register in
> the page fault path. But, even when the feature of setting faultaround
> pages with the old flag is disabled for a given cpu, we are still paying
> the cost of checking the register on every pagefault. This results in an
> explosion of vmexit events in KVM guests, which directly impacts the
> performance of virtualized workloads. For instance, running kernbench
> yields a 15% increase in system time solely due to the increased vmexit
> cycles.
>
> This patch avoids the extra cost by using the sanitized cached value.
> It should be safe to do so, since this register mustn't change for a
> given cpu.
>
> [1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31
>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Hi,
Considering the performance impact on kvm guests, unless someone
opposes, can we get this queued already for -rc5?
--
Gabriel Krisman Bertazi
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next prev parent reply other threads:[~2023-01-16 20:42 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-09 15:19 [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path Gabriel Krisman Bertazi
2023-01-11 8:34 ` Anshuman Khandual
2023-01-11 13:31 ` Gabriel Krisman Bertazi
2023-01-12 2:57 ` Anshuman Khandual
2023-01-12 3:05 ` Anshuman Khandual
2023-01-16 20:41 ` Gabriel Krisman Bertazi [this message]
2023-01-17 16:14 ` Will Deacon
2023-01-18 16:08 ` Gabriel Krisman Bertazi
2023-01-18 15:44 ` Catalin Marinas
2023-01-18 16:06 ` Gabriel Krisman Bertazi
2023-01-18 16:38 ` Will Deacon
2023-01-20 16:59 ` Catalin Marinas
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