Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <1469b131-cd76-e8bb-304b-73c59e81cb3b@redhat.com>
Date:   Tue, 30 Nov 2021 17:16:29 +0100
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Subject: Re: [PATCH 27/28] KVM: x86/mmu: Do remote TLB flush before dropping
 RCU in TDP MMU resched
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To:     Sean Christopherson <seanjc@google.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>
References: <20211120045046.3940942-1-seanjc@google.com>
 <20211120045046.3940942-28-seanjc@google.com>
 <df9d430c-2065-804b-2343-d4bcdb7b2464@redhat.com>
 <YaZG/NopJ7YaVUjD@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <YaZG/NopJ7YaVUjD@google.com>
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On 11/30/21 16:45, Sean Christopherson wrote:
>> 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.

Yeah, of course you're right.

Paolo