linux-kernel - Re: [PATCH v2] KVM: nVMX: Fix attempting to emulate "Acknowledge interrupt on exit" when there is no interrupt which L1 requires to inject to L2

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Date:   Thu, 3 Aug 2017 15:46:36 +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 attempting to emulate "Acknowledge
 interrupt on exit" when there is no interrupt which L1 requires to inject to
 L2

2017-07-31 19:25-0700, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> ------------[ cut here ]------------
>  WARNING: CPU: 5 PID: 2288 at arch/x86/kvm/vmx.c:11124 nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
>  CPU: 5 PID: 2288 Comm: qemu-system-x86 Not tainted 4.13.0-rc2+ #7
>  RIP: 0010:nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
> Call Trace:
>   vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
>   ? vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
>   kvm_arch_vcpu_ioctl_run+0x5dd/0x1be0 [kvm]
>   ? vmx_vcpu_load+0x1be/0x220 [kvm_intel]
>   ? kvm_arch_vcpu_load+0x62/0x230 [kvm]
>   kvm_vcpu_ioctl+0x340/0x700 [kvm]
>   ? kvm_vcpu_ioctl+0x340/0x700 [kvm]
>   ? __fget+0xfc/0x210
>   do_vfs_ioctl+0xa4/0x6a0
>   ? __fget+0x11d/0x210
>   SyS_ioctl+0x79/0x90
>   do_syscall_64+0x8f/0x750
>   ? trace_hardirqs_on_thunk+0x1a/0x1c
>   entry_SYSCALL64_slow_path+0x25/0x25
> 
> This can be reproduced by booting L1 guest w/ 'noapic' grub parameter, which 
> means that tells the kernel to not make use of any IOAPICs that may be present 
> in the system.
> 
> Actually external_intr variable in nested_vmx_vmexit() is the req_int_win 
> variable passed from vcpu_enter_guest() which means that the L0's userspace 
> requests an irq window. I observed the scenario (!kvm_cpu_has_interrupt(vcpu) &&
> L0's userspace reqeusts an irq window) is true, so there is no interrupt which 
> L1 requires to inject to L2, we should not attempt to emualte "Acknowledge 
> interrupt on exit" for the irq window requirement in this scenario.
> 
> This patch fixes it by not attempt to emulate "Acknowledge interrupt on exit"  
> if there is no L1 requirement to inject an interrupt to L2.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v1 -> v2:
>  * update patch description
>  * check nested_exit_intr_ack_set() first 
> 
>  arch/x86/kvm/vmx.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 2737343..c5a0ab5 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -11118,8 +11118,9 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
>  
>  	vmx_switch_vmcs(vcpu, &vmx->vmcs01);
>  
> -	if ((exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
> -	    && nested_exit_intr_ack_set(vcpu)) {

I've added a TODO comment so it's clearer that we should not be here if
there is no interrupt.

> +	if (nested_exit_intr_ack_set(vcpu) &&
> +		exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
> +		kvm_cpu_has_interrupt(vcpu)) {
>  		int irq = kvm_cpu_get_interrupt(vcpu);
>  		WARN_ON(irq < 0);
>  		vmcs12->vm_exit_intr_info = irq |

Changed the indentation to the original alignment.
Please don't use 1 tab -- the condition and body meld, which makes it
harder to read.  (2 tabs would be ok too.)

And the subject was way too long, so I changed it to
KVM: nVMX: Fix interrupt window request with "Acknowledge interrupt on exit"

Applied as it results in better behavior, even if it still is incorrect,
thanks.