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[PATCH] KVM: nVMX: fix MWAIT emulation for guests

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  [PATCH] KVM: nVMX: fix MWAIT emulation for guests Wanpeng Li <kernellwp@gmail.com> - 2017-05-04 11:40 +0200
    Re: [PATCH] KVM: nVMX: fix MWAIT emulation for guests Alexander Graf <agraf@suse.de> - 2017-05-04 11:50 +0200
    Re: [PATCH] KVM: nVMX: fix MWAIT emulation for guests Paolo Bonzini <pbonzini@redhat.com> - 2017-05-04 12:40 +0200
      Re: [PATCH] KVM: nVMX: fix MWAIT emulation for guests Wanpeng Li <kernellwp@gmail.com> - 2017-05-04 12:50 +0200

#1635599 — [PATCH] KVM: nVMX: fix MWAIT emulation for guests

From: Wanpeng Li <wanpeng.li@hotmail.com>

The kvm-unit-tests/vmx.flat fails against instruction(mwait) intercept test.

Test suite: instruction intercept
Unhandled exception 13 #GP at ip 0000000000402397
error_code=0000      rflags=00010012      cs=00000008
rax=0000000000000000 rcx=0000000004006172 rdx=0000000000402397 rbx=000000000041427d
rbp=0000000007ff8fdf rsi=0000000000000000 rdi=0000000000000004
 r8=000000000000000a  r9=00000000000003f8 r10=0000000000000000 r11=0000000000000000
 r12=0000000000000000 r13=0000000000000000 r14=0000000000000000 r15=0000000000000000
cr0=0000000080010031 cr2=0000000000000000 cr3=0000000007fff000 cr4=0000000000002020
cr8=0000000000000000
 	STACK: @402397 401102 400459

This testcase will run mwait instruction unconditional in L2 w/o check mwait 
cpuid. The vmlauch/vmresume emulation will merge L0's requirements and L1's 
requests, kvm-unit-tests doesn't set MWAIT/MONITOR EXITING bits for vmcs12.
w/o commit 668fffa3f838 ("kvm: better MWAIT emulation for guests"), the 
MWAIT/MONITOR EXITING bits in vmcs02 will be set since it is set in vmcs01 
though not in vmcs12. However, w/ the commit the bits will not be set in vmcs02 
since the bits are both not set in vmcs01(if kvm_mwait_in_guest() returns true) 
and vmcs12.

L2 should not occupy all the cpu time on L0 when L2 is idle, this patch fix it 
by unconditional set MWAIT/MONITOR EXTING bits against vmcs02.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: "Gabriel L. Somlo" <gsomlo@gmail.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/vmx.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 29940b6..104a0c6 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10098,6 +10098,13 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 	exec_control &= ~CPU_BASED_USE_IO_BITMAPS;
 	exec_control |= CPU_BASED_UNCOND_IO_EXITING;
 
+	/*
+	 * L2 should not occupy all the cpu time on L0 when L2
+	 * is idle, exit every time.
+	 */
+	exec_control |= CPU_BASED_MWAIT_EXITING |
+			CPU_BASED_MONITOR_EXITING;
+
 	vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, exec_control);
 
 	/* EXCEPTION_BITMAP and CR0_GUEST_HOST_MASK should basically be the
-- 
2.7.4

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#1635612

On 04.05.17 11:37, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> The kvm-unit-tests/vmx.flat fails against instruction(mwait) intercept test.
>
> Test suite: instruction intercept
> Unhandled exception 13 #GP at ip 0000000000402397
> error_code=0000      rflags=00010012      cs=00000008
> rax=0000000000000000 rcx=0000000004006172 rdx=0000000000402397 rbx=000000000041427d
> rbp=0000000007ff8fdf rsi=0000000000000000 rdi=0000000000000004
>  r8=000000000000000a  r9=00000000000003f8 r10=0000000000000000 r11=0000000000000000
>  r12=0000000000000000 r13=0000000000000000 r14=0000000000000000 r15=0000000000000000
> cr0=0000000080010031 cr2=0000000000000000 cr3=0000000007fff000 cr4=0000000000002020
> cr8=0000000000000000
>  	STACK: @402397 401102 400459
>
> This testcase will run mwait instruction unconditional in L2 w/o check mwait
> cpuid. The vmlauch/vmresume emulation will merge L0's requirements and L1's
> requests, kvm-unit-tests doesn't set MWAIT/MONITOR EXITING bits for vmcs12.
> w/o commit 668fffa3f838 ("kvm: better MWAIT emulation for guests"), the
> MWAIT/MONITOR EXITING bits in vmcs02 will be set since it is set in vmcs01
> though not in vmcs12. However, w/ the commit the bits will not be set in vmcs02
> since the bits are both not set in vmcs01(if kvm_mwait_in_guest() returns true)
> and vmcs12.
>
> L2 should not occupy all the cpu time on L0 when L2 is idle, this patch fix it
> by unconditional set MWAIT/MONITOR EXTING bits against vmcs02.

I don't understand - why not? If both L0 and L1 hypervisors allow MWAIT 
execution, why should we stop L2 from executing it?

We don't prevent L2 from running busy loops either, right?


Alex

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#1635627

On 04/05/2017 11:37, Wanpeng Li wrote:
> 
> The kvm-unit-tests/vmx.flat fails against instruction(mwait) intercept test.
> 
> Test suite: instruction intercept
> Unhandled exception 13 #GP at ip 0000000000402397
> error_code=0000      rflags=00010012      cs=00000008
> rax=0000000000000000 rcx=0000000004006172 rdx=0000000000402397 rbx=000000000041427d
> rbp=0000000007ff8fdf rsi=0000000000000000 rdi=0000000000000004
>  r8=000000000000000a  r9=00000000000003f8 r10=0000000000000000 r11=0000000000000000
>  r12=0000000000000000 r13=0000000000000000 r14=0000000000000000 r15=0000000000000000
> cr0=0000000080010031 cr2=0000000000000000 cr3=0000000007fff000 cr4=0000000000002020
> cr8=0000000000000000
>  	STACK: @402397 401102 400459

The problem is simply that the tests were not clearing eax/ecx/edx.

Update kvm-unit-tests.git, it includes the "[PATCH] vmx: correctly set
up MONITOR and MWAIT" from April 27 (message id
1493309532-14398-1-git-send-email-pbonzini@redhat.com).

Thanks,

Paolo

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#1635631

2017-05-04 18:37 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>
>
> On 04/05/2017 11:37, Wanpeng Li wrote:
>>
>> The kvm-unit-tests/vmx.flat fails against instruction(mwait) intercept test.
>>
>> Test suite: instruction intercept
>> Unhandled exception 13 #GP at ip 0000000000402397
>> error_code=0000      rflags=00010012      cs=00000008
>> rax=0000000000000000 rcx=0000000004006172 rdx=0000000000402397 rbx=000000000041427d
>> rbp=0000000007ff8fdf rsi=0000000000000000 rdi=0000000000000004
>>  r8=000000000000000a  r9=00000000000003f8 r10=0000000000000000 r11=0000000000000000
>>  r12=0000000000000000 r13=0000000000000000 r14=0000000000000000 r15=0000000000000000
>> cr0=0000000080010031 cr2=0000000000000000 cr3=0000000007fff000 cr4=0000000000002020
>> cr8=0000000000000000
>>       STACK: @402397 401102 400459
>
> The problem is simply that the tests were not clearing eax/ecx/edx.

You are right, thanks for pointing out this. :)

Regards,
Wanpeng Li

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