linux-kernel - Re: [PATCH v4 19/30] KVM: x86/mmu: Do remote TLB flush before dropping RCU in TDP MMU resched

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Message-ID: <YiFpJ8+fNtfEu3oO@google.com>
Date:   Fri, 4 Mar 2022 01:19:35 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v4 19/30] KVM: x86/mmu: Do remote TLB flush before
 dropping RCU in TDP MMU resched

On Thu, Mar 03, 2022, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> When yielding in the TDP MMU iterator, service any pending TLB flush
> before dropping RCU protections in anticipation of using the caller's RCU
> "lock" as a proxy for vCPUs in the guest.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
> Message-Id: <20220226001546.360188-19-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Reviewed-by: Mingwei Zhang <mizhang@...gle.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index c71debdbc732..3a866fcb5ea9 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -716,11 +716,11 @@ static inline bool __must_check tdp_mmu_iter_cond_resched(struct kvm *kvm,
>  		return false;
>  
>  	if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
> -		rcu_read_unlock();
> -
>  		if (flush)
>  			kvm_flush_remote_tlbs(kvm);
>  
> +		rcu_read_unlock();
> +
>  		if (shared)
>  			cond_resched_rwlock_read(&kvm->mmu_lock);
>  		else
> -- 
> 2.31.1
> 
>