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Message-ID: <YaZG/NopJ7YaVUjD@google.com>
Date: Tue, 30 Nov 2021 15:45:00 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.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 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.
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