linux-kernel - Re: [PATCH 18/24] kvm: x86/mmu: Use an rwlock for the x86 TDP MMU

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Date:   Wed, 20 Jan 2021 16:45:10 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 18/24] kvm: x86/mmu: Use an rwlock for the x86 TDP MMU

On Tue, Jan 12, 2021, Ben Gardon wrote:
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index ba296ad051c3..280d7cd6f94b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5471,6 +5471,11 @@ void kvm_mmu_init_vm(struct kvm *kvm)
>  
>  	kvm_mmu_init_tdp_mmu(kvm);
>  
> +	if (kvm->arch.tdp_mmu_enabled)
> +		rwlock_init(&kvm->arch.mmu_rwlock);
> +	else
> +		spin_lock_init(&kvm->arch.mmu_lock);

Rather than use different lock types, what if we always use a rwlock, but only
acquire it for read when handling page faults for TDP MMUs?  That would
significantly reduce the amount of boilerplate conditionals.

The fast paths for write_lock() and spin_lock() are nearly identical, and
I would hope any differences in the slow paths are hidden in the noise.

> +
>  	node->track_write = kvm_mmu_pte_write;
>  	node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
>  	kvm_page_track_register_notifier(kvm, node);
> @@ -6074,3 +6079,87 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
>  	if (kvm->arch.nx_lpage_recovery_thread)
>  		kthread_stop(kvm->arch.nx_lpage_recovery_thread);
>  }
> +
> +void kvm_mmu_lock_shared(struct kvm *kvm)
> +{
> +	WARN_ON(!kvm->arch.tdp_mmu_enabled);
> +	read_lock(&kvm->arch.mmu_rwlock);
> +}
> +
> +void kvm_mmu_unlock_shared(struct kvm *kvm)
> +{
> +	WARN_ON(!kvm->arch.tdp_mmu_enabled);
> +	read_unlock(&kvm->arch.mmu_rwlock);
> +}
> +
> +void kvm_mmu_lock_exclusive(struct kvm *kvm)
> +{
> +	WARN_ON(!kvm->arch.tdp_mmu_enabled);
> +	write_lock(&kvm->arch.mmu_rwlock);
> +}
> +
> +void kvm_mmu_unlock_exclusive(struct kvm *kvm)
> +{
> +	WARN_ON(!kvm->arch.tdp_mmu_enabled);
> +	write_unlock(&kvm->arch.mmu_rwlock);
> +}

I'm not a fan of all of these wrappers.  It's extra layers and WARNs, and
introduces terminology that differs from the kernel's locking terminology,
e.g. read vs. shared.  The WARNs are particularly wasteful as these all have
exactly one caller that explicitly checks kvm->arch.tdp_mmu_enabled.

Even if we don't unconditionally use the rwlock, I think I'd prefer to omit
these rwlock wrappers and instead use read/write_lock directly (and drop the
WARNs). 

> +
> +void kvm_mmu_lock(struct kvm *kvm)
> +{
> +	if (kvm->arch.tdp_mmu_enabled)
> +		kvm_mmu_lock_exclusive(kvm);
> +	else
> +		spin_lock(&kvm->arch.mmu_lock);
> +}
> +EXPORT_SYMBOL_GPL(kvm_mmu_lock);
> +
> +void kvm_mmu_unlock(struct kvm *kvm)
> +{
> +	if (kvm->arch.tdp_mmu_enabled)
> +		kvm_mmu_unlock_exclusive(kvm);
> +	else
> +		spin_unlock(&kvm->arch.mmu_lock);
> +}
> +EXPORT_SYMBOL_GPL(kvm_mmu_unlock);

These exports aren't needed, I don't see any callers in kvm_intel or kvm_amd.
That's a moot point if we use rwlock unconditionally.

> +