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From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Andrew Jones <drjones@redhat.com>
Cc: marc.zyngier@arm.com, Christoffer Dall <cdall@linaro.org>,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	pbonzini@redhat.com
Subject: Re: [PATCH v2 1/9] KVM: add kvm_request_pending
Date: Wed, 5 Apr 2017 15:10:50 +0200	[thread overview]
Message-ID: <20170405131049.GD6369@potion> (raw)
In-Reply-To: <20170404164120.xvlvyebvcqoci5cu@kamzik.brq.redhat.com>

2017-04-04 18:41+0200, Andrew Jones:
> On Tue, Apr 04, 2017 at 05:30:14PM +0200, Christoffer Dall wrote:
>> On Fri, Mar 31, 2017 at 06:06:50PM +0200, Andrew Jones wrote:
>> > From: Radim Krčmář <rkrcmar@redhat.com>
>> > 
>> > A first step in vcpu->requests encapsulation.
>> 
>> Could we have a note here on why we need to access vcpu->requests using
>> READ_ONCE now?
> 
> Sure, maybe we should put the note as a comment above the read in
> kvm_request_pending().  Something like
> 
>  /*
>   * vcpu->requests reads may appear in sequences that have strict
>   * data or control dependencies.  Use READ_ONCE() to ensure the
>   * compiler does not do anything that breaks the required ordering.
>   */
> 
> Radim?

Uses of vcpu->requests should already have barriers that take care of
the ordering.  I think the main reason for READ_ONCE() is to tell
programmers that requests are special, but predictable.

READ_ONCE() is not necessary in any use I'm aware of, but there is no
harm in telling the compiler that vcpu->requests are what we think they
are ...

 /*
  * vcpu->requests are a lockless synchronization mechanism, where
  * memory barriers are necessary for correct behavior, see
  * Documentation/virtual/kvm/vcpu-requests.rst.
  *
  * READ_ONCE() is not necessary for correctness, but simplifies
  * reasoning by constricting the generated code.
  */

I considered READ_ONCE() to be self-documenting. :)
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  reply	other threads:[~2017-04-05 13:10 UTC|newest]

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-31 16:06 [PATCH v2 0/9] KVM: arm/arm64: race fixes and vcpu requests Andrew Jones
2017-03-31 16:06 ` [PATCH v2 1/9] KVM: add kvm_request_pending Andrew Jones
2017-04-04 15:30   ` Christoffer Dall
2017-04-04 16:41     ` Andrew Jones
2017-04-05 13:10       ` Radim Krčmář [this message]
2017-04-05 17:39         ` Christoffer Dall
2017-04-05 18:30           ` Paolo Bonzini
2017-04-05 20:20           ` Radim Krčmář
2017-04-06 12:02             ` Andrew Jones
2017-04-06 14:37               ` Christoffer Dall
2017-04-06 15:08                 ` Andrew Jones
2017-04-07 15:33                   ` Paolo Bonzini
2017-04-08 18:19                     ` Christoffer Dall
2017-04-06 14:25             ` Christoffer Dall
2017-04-07 13:15               ` Radim Krčmář
2017-04-08 18:23                 ` Christoffer Dall
2017-04-08 19:32                   ` Paolo Bonzini
2017-04-11 21:06                     ` Radim Krčmář
2017-03-31 16:06 ` [PATCH v2 2/9] KVM: Add documentation for VCPU requests Andrew Jones
2017-04-04 15:24   ` Christoffer Dall
2017-04-04 17:06     ` Andrew Jones
2017-04-04 17:23       ` Christoffer Dall
2017-04-04 17:36         ` Paolo Bonzini
2017-04-05 14:11         ` Radim Krčmář
2017-04-05 17:45           ` Christoffer Dall
2017-04-05 18:29             ` Paolo Bonzini
2017-04-05 20:46               ` Radim Krčmář
2017-04-06 14:29                 ` Christoffer Dall
2017-04-07 11:44                   ` Paolo Bonzini
2017-04-06 14:27               ` Christoffer Dall
2017-04-06 10:18   ` Christian Borntraeger
2017-04-06 12:08     ` Andrew Jones
2017-04-06 12:29     ` Radim Krčmář
2017-03-31 16:06 ` [PATCH v2 3/9] KVM: arm/arm64: prepare to use vcpu requests Andrew Jones
2017-04-04 15:34   ` Christoffer Dall
2017-04-04 17:06     ` Andrew Jones
2017-03-31 16:06 ` [PATCH v2 4/9] KVM: arm/arm64: replace vcpu->arch.pause with a vcpu request Andrew Jones
2017-04-04 13:39   ` Marc Zyngier
2017-04-04 14:47     ` Andrew Jones
2017-04-04 14:51       ` Paolo Bonzini
2017-04-04 15:05         ` Marc Zyngier
2017-04-04 17:07         ` Andrew Jones
2017-04-04 16:04   ` Christoffer Dall
2017-04-04 16:24     ` Paolo Bonzini
2017-04-04 17:19       ` Christoffer Dall
2017-04-04 17:35         ` Paolo Bonzini
2017-04-04 17:57           ` Christoffer Dall
2017-04-04 18:15             ` Paolo Bonzini
2017-04-04 18:38               ` Christoffer Dall
2017-04-04 18:18           ` Andrew Jones
2017-04-04 18:59             ` Paolo Bonzini
2017-04-04 17:57     ` Andrew Jones
2017-04-04 19:04       ` Christoffer Dall
2017-04-04 20:10         ` Paolo Bonzini
2017-04-05  7:09           ` Christoffer Dall
2017-04-05 11:37             ` Paolo Bonzini
2017-04-06 14:14               ` Christoffer Dall
2017-04-07 11:47                 ` Paolo Bonzini
2017-04-08  8:35                   ` Christoffer Dall
2017-03-31 16:06 ` [PATCH v2 5/9] KVM: arm/arm64: replace vcpu->arch.power_off " Andrew Jones
2017-04-04 17:37   ` Christoffer Dall
2017-03-31 16:06 ` [PATCH v2 6/9] KVM: arm/arm64: use a vcpu request on irq injection Andrew Jones
2017-04-04 17:42   ` Christoffer Dall
2017-04-04 18:27     ` Andrew Jones
2017-04-04 18:59     ` Paolo Bonzini
2017-04-04 18:51   ` Paolo Bonzini
2017-03-31 16:06 ` [PATCH v2 7/9] KVM: arm/arm64: PMU: remove request-less vcpu kick Andrew Jones
2017-04-04 17:46   ` Christoffer Dall
2017-04-04 18:29     ` Andrew Jones
2017-04-04 19:35       ` Christoffer Dall
2017-03-31 16:06 ` [PATCH v2 8/9] KVM: arm/arm64: fix race in kvm_psci_vcpu_on Andrew Jones
2017-04-04 19:42   ` Christoffer Dall
2017-04-05  8:35     ` Andrew Jones
2017-04-05  8:50       ` Christoffer Dall
2017-04-05  9:12         ` Andrew Jones
2017-04-05  9:30           ` Christoffer Dall
2017-03-31 16:06 ` [PATCH v2 9/9] KVM: arm/arm64: avoid race by caching MPIDR Andrew Jones
2017-04-04 19:44   ` Christoffer Dall
2017-04-05  8:50     ` Andrew Jones
2017-04-05 11:03       ` Christoffer Dall
2017-04-05 11:14         ` Andrew Jones
2017-04-03 15:28 ` [PATCH v2 0/9] KVM: arm/arm64: race fixes and vcpu requests Christoffer Dall
2017-04-03 17:11   ` Paolo Bonzini
2017-04-04  7:27   ` Andrew Jones
2017-04-04 16:05     ` Christoffer Dall

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