linux-kernel - Re: [PATCH 27/28] KVM: x86/mmu: Do remote TLB flush before dropping RCU in TDP MMU resched

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Message-ID: <1469b131-cd76-e8bb-304b-73c59e81cb3b@redhat.com>
Date:   Tue, 30 Nov 2021 17:16:29 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 27/28] KVM: x86/mmu: Do remote TLB flush before dropping
 RCU in TDP MMU resched

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