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From: James Morse <james.morse@arm.com>
To: Gavin Shan <gshan@redhat.com>, kvmarm@lists.cs.columbia.edu
Cc: maz@kernel.org, will@kernel.org, shan.gavin@gmail.com,
	pbonzini@redhat.com
Subject: Re: [PATCH 0/6] Support Asynchronous Page Fault
Date: Fri, 23 Oct 2020 17:54:06 +0100	[thread overview]
Message-ID: <4dcaea7a-a4d1-9bf3-eb95-ea9d8826ad99@arm.com> (raw)
In-Reply-To: <20200818011319.91777-1-gshan@redhat.com>

Hi Gavin,

I think this series would benefit from being in smaller pieces. I got lost in patch 4 for
quite a while. Suggestion on where to do that in patch 4.


On 18/08/2020 02:13, Gavin Shan wrote:
> There are two stages of page fault. The guest kernel is responsible
> for handling stage one page fault, while the host kernel is to take
> care of the stage two page fault. When page fault is triggered because
> of stage two page fault, the guest is suspended until the requested
> memory (page) is populated. Sometimes, the cost to populate the requested
> page isn't cheap and can take hundreds of milliseconds in extreme
> cases. This impacts the overall guest's performance.

You really need to use postcopy live migration to justify these changes. Otherwise the
story here is "over-commited hosts suck", which I don't think anyone cares about.


> This series introduces the feature (asynchronous page fault) to resolve
> the issue and improve the guest's performance. It depends on the series
> to support SDEI virtualization and refactoring SDEI client driver.

SDEI gives you an NMI ... which you use to set a TIF flag. This can only work reliably for
user-space. So much so that you have code in the hypervisor to only deliver the NMI ...
when in user-space.
The only reason you would need an NMI is to interrupt interrupts-masked code. Linux can't
reschedule when this is the case.

I can only conclude, you really don't need an NMI here.


Why couldn't we use an IRQ here, it would be a lot simpler? ... the reason is the arm
architecture can't guarantee us that we take the irq when there is also a stage2 fault for
the first instruction.
I reckon we can work around this in the hypervisor:
https://lore.kernel.org/r/20201023165108.15061-1-james.morse@arm.com


My problem with SDEI is, you don't really need an NMI, and it creates extra in-kernel
state that has to be migrated. I think having this state in the kernel complicates the
user-space handling of SIGBUS_MCEERR_AO signals that don't get delivered to a vCPU thread.


Thanks,

James
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  parent reply	other threads:[~2020-10-23 16:54 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-18  1:13 [PATCH 0/6] Support Asynchronous Page Fault Gavin Shan
2020-08-18  1:13 ` [PATCH 1/6] arm64: Probe for the presence of KVM hypervisor services during boot Gavin Shan
2020-08-18  1:13 ` [PATCH 2/6] arm/arm64: KVM: Advertise KVM UID to guests via SMCCC Gavin Shan
2020-08-18  1:13 ` [PATCH 3/6] kvm/arm64: Export kvm_handle_user_mem_abort() with prefault mode Gavin Shan
2020-10-23 16:54   ` James Morse
2020-11-02  5:30     ` Gavin Shan
2020-08-18  1:13 ` [PATCH 4/6] arm64/kvm: Support async page fault Gavin Shan
2020-10-23 17:01   ` James Morse
2020-11-02  7:19     ` Gavin Shan
2020-08-18  1:13 ` [PATCH 5/6] drivers/acpi: Import ACPI APF table Gavin Shan
2020-10-23 16:55   ` James Morse
2020-08-18  1:13 ` [PATCH 6/6] arm64/kernel: Support async page fault Gavin Shan
2020-10-23 16:54 ` James Morse [this message]
2020-10-31 14:18   ` [PATCH 0/6] Support Asynchronous Page Fault Paolo Bonzini
2020-11-02  5:23   ` Gavin Shan

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