linux-kernel - Re: [PATCH] KVM: x86/mmu: Add '__func__' in rmap

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Date:   Wed, 27 Jan 2021 10:28:26 -0800
From:   Joe Perches <joe@...ches.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Stephen Zhang <stephenzhangzsd@...il.com>, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Add '__func__' in rmap_printk()

On Wed, 2021-01-27 at 19:23 +0100, Paolo Bonzini wrote:
> On 27/01/21 18:25, Joe Perches wrote:
> > 
> > -#ifdef MMU_DEBUG
> > -bool dbg = 0;
> > -module_param(dbg, bool, 0644);
> > -#endif
> > -
> >  #define PTE_PREFETCH_NUM		8
> >  
> > 
> >  #define PT32_LEVEL_BITS 10
> > @@ -844,17 +839,17 @@ static int pte_list_add(struct kvm_vcpu *vcpu, u64 *spte,
> >  	int i, count = 0;
> >  
> > 
> >  	if (!rmap_head->val) {
> > -		rmap_printk("pte_list_add: %p %llx 0->1\n", spte, *spte);
> > +		pr_debug("%p %llx 0->1\n", spte, *spte);
> >  		rmap_head->val = (unsigned long)spte;
> >  	} else if (!(rmap_head->val & 1)) {
> > -		rmap_printk("pte_list_add: %p %llx 1->many\n", spte, *spte);
> > +		pr_debug("%p %llx 1->many\n", spte, *spte);
> >  		desc = mmu_alloc_pte_list_desc(vcpu);
> 
> This would be extremely slow.  These messages are even too verbose for 
> tracepoints.

There's no real object change.

It's not enabled unless DEBUG is defined (and it's not enabled by default)
or CONFIG_DYNAMIC_DEBUG is enabled and then dynamic_debug jump points are
used when not enabled so I think any slowdown, even when dynamic_debug is
enabled is trivial.