linux-kernel - Re: [PATCH v2] KVM: nVMX: Fix trying to cancel vmlauch/vmresume

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Date:   Mon, 21 Aug 2017 18:20:53 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2] KVM: nVMX: Fix trying to cancel vmlauch/vmresume

2017-08-18 07:11-0700, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> ------------[ cut here ]------------
> WARNING: CPU: 7 PID: 3861 at /home/kernel/ssd/kvm/arch/x86/kvm//vmx.c:11299 nested_vmx_vmexit+0x176e/0x1980 [kvm_intel]
> CPU: 7 PID: 3861 Comm: qemu-system-x86 Tainted: G        W  OE   4.13.0-rc4+ #11
> RIP: 0010:nested_vmx_vmexit+0x176e/0x1980 [kvm_intel]
> Call Trace:
>  ? kvm_multiple_exception+0x149/0x170 [kvm]
>  ? handle_emulation_failure+0x79/0x230 [kvm]
>  ? load_vmcs12_host_state+0xa80/0xa80 [kvm_intel]
>  ? check_chain_key+0x137/0x1e0
>  ? reexecute_instruction.part.168+0x130/0x130 [kvm]
>  nested_vmx_inject_exception_vmexit+0xb7/0x100 [kvm_intel]
>  ? nested_vmx_inject_exception_vmexit+0xb7/0x100 [kvm_intel]
>  vmx_queue_exception+0x197/0x300 [kvm_intel]
>  kvm_arch_vcpu_ioctl_run+0x1b0c/0x2c90 [kvm]
>  ? kvm_arch_vcpu_runnable+0x220/0x220 [kvm]
>  ? preempt_count_sub+0x18/0xc0
>  ? restart_apic_timer+0x17d/0x300 [kvm]
>  ? kvm_lapic_restart_hv_timer+0x37/0x50 [kvm]
>  ? kvm_arch_vcpu_load+0x1d8/0x350 [kvm]
>  kvm_vcpu_ioctl+0x4e4/0x910 [kvm]
>  ? kvm_vcpu_ioctl+0x4e4/0x910 [kvm]
>  ? kvm_dev_ioctl+0xbe0/0xbe0 [kvm]
> 
> The flag "nested_run_pending", which can override the decision of which should run 
> next, L1 or L2. nested_run_pending=1 means that we *must* run L2 next, not L1. This 
> is necessary in particular when L1 did a VMLAUNCH of L2 and therefore expects L2 to 
> be run (and perhaps be injected with an event it specified, etc.). Nested_run_pending 
> is especially intended to avoid switching  to L1 in the injection decision-point.
> 
> I catch this in the queue exception path, this patch fixes it by requesting 
> an immediate VM exit from L2 and keeping the exception for L1 pending for a 
> subsequent nested VM exit.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -6356,8 +6356,8 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>  			kvm_update_dr7(vcpu);
>  		}

Hm, we shouldn't execute the code above if exception won't be injected.

>  
> -		kvm_x86_ops->queue_exception(vcpu);
> -		return 0;

vmx_complete_interrupts() assumes that the exception is always injected,
so it would be dropped by kvm_clear_exception_queue().

I'm starting to wonder whether getting rid of nested_run_pending
wouldn't be nicer.

> +		r = kvm_x86_ops->queue_exception(vcpu);
> +		return r;
>  	}
>  
>  	if (vcpu->arch.nmi_injected) {
> -- 
> 2.7.4
>