Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set

[Radim Krcmar]

2017-11-09 20:43+0800, Wanpeng Li:
> 2017-11-07 4:26 GMT+08:00 Eduardo Valentin <eduval@xxxxxxxxxx>:
> > 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.
> >
> > With this patch, when in autoselect mode, the guest will
> > use the default spinlock implementation based on host feature
> > flags as follows:
> >
> > 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
> >
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Waiman Long <longman@xxxxxxxxxx>
> > Cc: kvm@xxxxxxxxxxxxxxx
> > Cc: linux-doc@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: Jan H. Schoenherr <jschoenh@xxxxxxxxx>
> > Cc: Anthony Liguori <aliguori@xxxxxxxxxx>
> > Suggested-by: Matt Wilson <msw@xxxxxxxxxx>
> > Signed-off-by: Eduardo Valentin <eduval@xxxxxxxxxx>
> > ---
> 
> You should also add a cpuid flag in kvm part.

It is better without that.  The flag has no dependency on KVM (kernel
hypervisor) code.