lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Fri, 3 Nov 2017 11:09:53 +0100
From:   Paolo Bonzini <>
To:     Eduardo Valentin <>
Cc:, Matt Wilson <>,
        Jonathan Corbet <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
        "H. Peter Anvin" <>,,
        Peter Zijlstra <>,
        Waiman Long <>,,,,
        "Jan H . Schoenherr" <>,
        Anthony Liguori <>
Subject: Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when

On 02/11/2017 19:43, Eduardo Valentin wrote:
> On Thu, Nov 02, 2017 at 07:24:16PM +0100, Paolo Bonzini wrote:
>> On 02/11/2017 19:08, Eduardo Valentin wrote:
>>> On Thu, Nov 02, 2017 at 06:56:46PM +0100, Paolo Bonzini wrote:
>>>> On 02/11/2017 18:45, Eduardo Valentin wrote:
>>>>> Currently, the existing qspinlock implementation will fallback to
>>>>> test-and-set if the hypervisor has not set the PV_UNHALT flag.
>>>>> This patch gives the opportunity to guest kernels to select
>>>>> between test-and-set and the regular queueu fair lock implementation
>>>>> based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
>>>>> flag is not set, the code will still fall back to test-and-set,
>>>>> but when the PV_DEDICATED flag is set, the code will use
>>>>> the regular queue spinlock implementation.
>>>> Have you seen Waiman's series that lets you specify this on the guest
>>>> command line instead?  Would this be acceptable for your use case?
>>> No, can you please share a link to it? is it already merged to tip/master?
>> [PATCH-tip v2 0/2] x86/paravirt: Enable users to choose PV lock type
>>>> (In other words, is there a difference for you between making the host
>>>> vs. guest administrator toggle the feature?  "" means you are
>>>> the host admin, how would you use it?)
>>> The way I think of this is this is a flag set by host side so the
>>> guest adapts accordingly.
>>> If the admin in guest side wants to ignore what the host is
>>> flagging, that is a different story.
>> Okay, this makes sense.  But perhaps it should be a separate CPUID leaf,
>> such as "configuration hints", rather than properly a feature.
> Oh OK, you don't think this starts to deviate from the feature concept.
> But would the PV_UNHALT also go to "configuration hints" bucket?

PV_UNHALT says whether the pvqspinlock API is available, PV_DEDICATED
says whether it should be used.

> Another way to see this is we have three locking feature options to select from,
> so we need at least two bits here.

PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock
PV_DEDICATED = 0, PV_UNHALT = 0: default is tas

What do you think?


Powered by blists - more mailing lists