linux-kernel - Re: [PATCH] KVM: x86: make KVM_REQ_NMI request iff NMI pending for vcpu

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Message-ID: <ZcKdhuE2yNednYPD@google.com>
Date: Tue, 6 Feb 2024 12:58:46 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Dongli Zhang <dongli.zhang@...cle.com>
Cc: Prasad Pandit <ppandit@...hat.com>, Prasad Pandit <pjp@...oraproject.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: make KVM_REQ_NMI request iff NMI pending for vcpu

On Tue, Feb 06, 2024, Dongli Zhang wrote:
> Hi Prasad,
> 
> On 1/2/24 23:53, Prasad Pandit wrote:
> > From: Prasad Pandit <pjp@...oraproject.org>
> > 
> > kvm_vcpu_ioctl_x86_set_vcpu_events() routine makes 'KVM_REQ_NMI'
> > request for a vcpu even when its 'events->nmi.pending' is zero.
> > Ex:
> >     qemu_thread_start
> >      kvm_vcpu_thread_fn
> >       qemu_wait_io_event
> >        qemu_wait_io_event_common
> >         process_queued_cpu_work
> >          do_kvm_cpu_synchronize_post_init/_reset
> >           kvm_arch_put_registers
> >            kvm_put_vcpu_events (cpu, level=[2|3])
> > 
> > This leads vCPU threads in QEMU to constantly acquire & release the
> > global mutex lock, delaying the guest boot due to lock contention.
> 
> Would you mind sharing where and how the lock contention is at QEMU space? That
> is, how the QEMU mutex lock is impacted by KVM KVM_REQ_NMI?
> 
> Or you meant line 3031 at QEMU side?

Yeah, something like that.  Details in this thread.

https://lore.kernel.org/all/CAE8KmOyffXD4k69vRAFwesaqrBGzFY3i+kefbkHcQf4=jNYzOA@mail.gmail.com

> 2858 int kvm_cpu_exec(CPUState *cpu)
> 2859 {
> 2860     struct kvm_run *run = cpu->kvm_run;
> 2861     int ret, run_ret;
> ... ...
> 3023         default:
> 3024             DPRINTF("kvm_arch_handle_exit\n");
> 3025             ret = kvm_arch_handle_exit(cpu, run);
> 3026             break;
> 3027         }
> 3028     } while (ret == 0);
> 3029
> 3030     cpu_exec_end(cpu);
> 3031     qemu_mutex_lock_iothread();
> 3032
> 3033     if (ret < 0) {
> 3034         cpu_dump_state(cpu, stderr, CPU_DUMP_CODE);
> 3035         vm_stop(RUN_STATE_INTERNAL_ERROR);
> 3036     }
> 3037
> 3038     qatomic_set(&cpu->exit_request, 0);
> 3039     return ret;
> 3040 }