Re: [PATCH v3 1/4] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks

[Zhenzhong Duan]

On 2019/10/3 1:10, Sean Christopherson wrote:

> On Mon, Sep 30, 2019 at 08:44:36PM +0800, Zhenzhong Duan wrote:
> > There are cases where a guest tries to switch spinlocks to bare metal
> > behavior (e.g. by setting "xen_nopvspin" on XEN platform and
> > "hv_nopvspin" on HYPER_V).
> >
> > That feature is missed on KVM, add a new parameter "nopvspin" to disable
> > PV spinlocks for KVM guest.
> >
> > This new parameter is also used to replace "xen_nopvspin" and
> > "hv_nopvspin".
> This is confusing as there are no Xen or Hyper-V changes in this patch.
> Please make it clear that you're talking about future patches, e.g.:
>
>    The new 'nopvspin' parameter will also replace Xen and Hyper-V specific
>    parameters in future patches.

Will fix

> > The global variable pvspin isn't defined as __initdata as it's used at
> > runtime by XEN guest.
> Same comment as above regarding what this patch is doing versus what will
> be done in the future.  Arguably you should even mark it __initdata in
> this patch and deal with conflict in the Xen patch, e.g. use it only to
> set the existing xen_pvspin variable.

Will fix

> > Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>

......snip

> >   /**
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index e820568..a4f108d 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -842,6 +842,13 @@ void __init kvm_spinlock_init(void)
> >   	if (num_possible_cpus() == 1)
> >   		return;
> >   
> > +	if (!pvspin) {
> > +		pr_info("PV spinlocks disabled\n");
> > +		static_branch_disable(&virt_spin_lock_key);
> > +		return;
> > +	}
> > +	pr_info("PV spinlocks enabled\n");
> These prints could be confusing as KVM also disables PV spinlocks when it
> sees KVM_HINTS_REALTIME.

What about below:

pr_info("PV spinlocks disabled forced by \"nopvspin\" parameter.\n");

Or you prefer separate print for each disabling like below?

        /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
        if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
                pr_info("PV spinlocks disabled, KVM_FEATURE_PV_UNHALT feature needed.\n");
                return;
        }

        if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
                pr_info("PV spinlocks disabled, having non-preemption hints.\n");
                return;
        }

        /* Don't use the pvqspinlock code if there is only 1 vCPU. */
        if (num_possible_cpus() == 1) {
                pr_info("PV spinlocks disabled on UP.\n");
                return;
        }
	if (!pvspin) {
		pr_info("PV spinlocks disabled forced by \"nopvspin\" parameter.\n");
		static_branch_disable(&virt_spin_lock_key);
		return;
	}
	pr_info("PV spinlocks enabled\n");

> > +
> >   	__pv_init_lock_hash();
> >   	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
> >   	pv_ops.lock.queued_spin_unlock =
> > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> > index 2473f10..945b510 100644
> > --- a/kernel/locking/qspinlock.c
> > +++ b/kernel/locking/qspinlock.c
> > @@ -580,4 +580,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> >   #include "qspinlock_paravirt.h"
> >   #include "qspinlock.c"
> >   
> > +bool pvspin = true;
> This can be __ro_after_init, or probably better __initdata and have Xen
> snapshot the value for its use case.

I will use __initdata

> Personal preference: I'd invert the bool and name it nopvspin to make it
> easier to connect the variable to the kernel param.

OK, will do that. Thanks for review for all the patches.

Zhenzhong