Date:   Tue, 30 Nov 2021 15:45:00 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Hou Wenlong <houwenlong93@linux.alibaba.com>,
        Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH 27/28] KVM: x86/mmu: Do remote TLB flush before dropping
 RCU in TDP MMU resched
Message-ID: <YaZG/NopJ7YaVUjD@google.com>
References: <20211120045046.3940942-1-seanjc@google.com>
 <20211120045046.3940942-28-seanjc@google.com>
 <df9d430c-2065-804b-2343-d4bcdb7b2464@redhat.com>
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In-Reply-To: <df9d430c-2065-804b-2343-d4bcdb7b2464@redhat.com>
Precedence: bulk

On Tue, Nov 30, 2021, Paolo Bonzini wrote:
> On 11/20/21 05:50, Sean Christopherson wrote:
> >   	if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
> > -		rcu_read_unlock();
> > -
> >   		if (flush)
> >   			kvm_flush_remote_tlbs(kvm);
> > +		rcu_read_unlock();
> > +
> 
> Couldn't this sleep in kvm_make_all_cpus_request, whilst in an RCU read-side
> critical section?

No.  And if kvm_make_all_cpus_request() can sleep, the TDP MMU is completely hosed
as tdp_mmu_zap_spte_atomic() and handle_removed_tdp_mmu_page() currently call
kvm_flush_remote_tlbs_with_range() while under RCU protection.

kvm_make_all_cpus_request_except() disables preemption via get_cpu(), and
smp_call_function() doubles down on disabling preemption as the inner helpers
require preemption to be disabled, so anything below them should complain if
there's a might_sleep().  hv_remote_flush_tlb_with_range() takes a spinlock, so
nothing in there should be sleeping either.