Re: [PATCH v4] x86/paravirt: Disable virt spinlock on bare metal

[Chen Yu <yu.c.chen@intel.com>]

On 2024-08-02 at 09:27:32 +0800, maobibo wrote:
> Hi Chenyu,
> 
> On 2024/8/1 下午10:40, Chen Yu wrote:
> > Hi Bibo,
> > 
> > On 2024-08-01 at 16:00:19 +0800, maobibo wrote:
> > > Chenyu,
> > > 
> > > I do not know much about x86, just give some comments(probably incorrected)
> > > from the code.
> > > 
> > > On 2024/7/29 下午2:52, Chen Yu wrote:
> > > > X86_FEATURE_HYPERVISOR         Y    Y    Y     N
> > > > CONFIG_PARAVIRT_SPINLOCKS      Y    Y    N     Y/N
> > > > PV spinlock                    Y    N    N     Y/N
> > > > 
> > > > virt_spin_lock_key             N    N    Y     N
> > > > 
> > > > -DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
> > > > +DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
> > > 
> > > @@ -87,7 +87,7 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
> > >   {
> > >          int val;
> > > 
> > > -       if (!static_branch_likely(&virt_spin_lock_key))
> > > +       if (!static_branch_unlikely(&virt_spin_lock_key))
> > >                  return false;
> > > 
> > > Do we need change it with static_branch_unlikely() if value of key is false
> > > by fault?
> > 
> > My understanding is that, firstly, whether it is likely() or unlikely()
> > depends on the 'expected' value of the key, rather than its default
> > initialized value. The compiler can arrange the if 'jump' condition to
> > avoid the overhead of branch jump(to keep the instruction pipeline)
> > as much as possible. Secondly, before this patch, the 'expected' value
> > of virt_spin_lock_key is 'true', so I'm not sure if we should change
> > its behavior. Although it seems that in most VM guest, with para-virt
> > spinlock enabled, this key should be false at most time, but just in
> > case of any regression...
> yes, it does not inflect the result, it is a trivial thing and depends on
> personal like or dislike.
> 
> > 
> > > >    /*
> > > >     * Shortcut for the queued_spin_lock_slowpath() function that allows
> > > > diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> > > > index 5358d43886ad..fec381533555 100644
> > > > --- a/arch/x86/kernel/paravirt.c
> > > > +++ b/arch/x86/kernel/paravirt.c
> > > > @@ -51,13 +51,12 @@ DEFINE_ASM_FUNC(pv_native_irq_enable, "sti", .noinstr.text);
> > > >    DEFINE_ASM_FUNC(pv_native_read_cr2, "mov %cr2, %rax", .noinstr.text);
> > > >    #endif
> > > > -DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
> > > > +DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
> > > >    void __init native_pv_lock_init(void)
> > > >    {
> > > > -	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
> > > > -	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
> > > > -		static_branch_disable(&virt_spin_lock_key);
> > > > +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> > > > +		static_branch_enable(&virt_spin_lock_key);
> > > >    }
> > > 
> > >  From my point, the sentence static_branch_disable(&virt_spin_lock_key) can
> > > be removed in file arch/x86/xen/spinlock.c and arch/x86/kernel/kvm.c, since
> > > function native_smp_prepare_boot_cpu() is already called by
> > > xen_smp_prepare_boot_cpu() and kvm_smp_prepare_boot_cpu().
> > > 
> > 
> > The key is enabled by native_smp_prepare_boot_cpu() for VM guest as
> > the initial value(default to true). And later either arch/x86/xen/spinlock.c
> > or arch/x86/kernel/kvm.c disable this key in a on-demand manner.
> I understand that you only care about host machine and do not want to change
> behavior of VM. Only that from the view of VM, there are two conditions such
> as:
> 
> 1. If option CONFIG_PARAVIRT_SPINLOCKS is disabled, virt_spin_lock_key is
> disabled with your patch. VM will use test-set spinlock rather than
> qspinlock to avoid the over-preemption of native qspinlock, just the same
> with commit 2aa79af64263. And it is the same for all the hypervisor types.
> 
> 2. If option CONFIG_PARAVIRT_SPINLOCKS is enable, pv spinlock cannot be used
> because some reasons, such as host hypervisor has no KVM_FEATURE_PV_UNHALT
> feature or nopvspin kernel parameter is added. The behavior to use test-set
> spinlock or native qspinlock is different on different hypervisor types.
> 
> Even on KVM hypervisor, if KVM_FEATURE_PV_UNHALT is not supported, test-set
> spinlock will be used on VM; For nopvspin kernel parameter, native spinlock
> is used on VM. What is the rule for this? :)
>

If CONFIG_PARAVIRT_SPINLOCKS is enabled, the test-set spinlock has nothing to do
with the lock path, because if pv_enabled() is true, it will skip the
test-set spinlock and go to pv_queue section. If for some reason the pv spinlock
can not be used because KVM_FEATURE_PV_UNHALT is not supported, it will fall into
the default qpinlock without pv-qspinlock(no pv_wait hook because it is NULL).

thanks,
Chenyu