Re: Nested VMX - L1 hangs on running L2

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jul 08, 2011 at 02:40:53PM -0400, Bandan Das wrote:
> I have already discussed this a bit with Nadav but hoping someone 
> else has any other ideas/clues/suggestions/comments. With recent versions of the 
> kernel (The last I tried is 3.0-rc5 with nVMX patches already merged), my L1 guest 
> always hangs when I start L2. 
> 
> My setup : The host, L1 and L2 all are FC15 with the host running 3.0-rc5. When L1 is up 
> and running, I start L2 from L1. Within a minute or two, both L1 and L2 hang. Although, if
> if I run tracing on the host, I see :
> 
> ...
> qemu-kvm-19756 [013] 153774.856178: kvm_exit: reason APIC_ACCESS rip 0xffffffff81025098 info 1380 0
> qemu-kvm-19756 [013] 153774.856189: kvm_exit: reason VMREAD rip 0xffffffffa00d5127 info 0 0
> qemu-kvm-19756 [013] 153774.856191: kvm_exit: reason VMREAD rip 0xffffffffa00d5127 info 0 0
> ...
> 
> My point being that I only see kvm_exit messages but no kvm_entry. Does this mean that the VCPUs
> are somehow stuck in L2 ?
> 
> Anyway, since this setup was running fine for me on older kernels, and I couldn't
> identify any significant changes in nVMX, I sifted through the other KVM changes and found this :
> 
> --
> commit 1aa8ceef0312a6aae7dd863a120a55f1637b361d
> Author: Nikola Ciprich <extmaillist@linuxbox.cz>
> Date:   Wed Mar 9 23:36:51 2011 +0100
> 
>     KVM: fix kvmclock regression due to missing clock update
>     
>     commit 387b9f97750444728962b236987fbe8ee8cc4f8c moved kvm_request_guest_time_update(vcpu),
>     breaking 32bit SMP guests using kvm-clock. Fix this by moving (new) clock update function
>     to proper place.
>     
>     Signed-off-by: Nikola Ciprich <nikola.ciprich@linuxbox.cz>
>     Acked-by: Zachary Amsden <zamsden@redhat.com>
>     Signed-off-by: Avi Kivity <avi@redhat.com>
> 
> index 01f08a6..f1e4025 100644 (file)
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2127,8 +2127,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>                 if (check_tsc_unstable()) {
>                         kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
>                         vcpu->arch.tsc_catchup = 1;
> -                       kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>                 }
> +               kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>                 if (vcpu->cpu != cpu)
>                         kvm_migrate_timers(vcpu);
>                 vcpu->cpu = cpu;
> --
> 
> If I revert this change, my L1/L2 guests run fine. This ofcourse, just hides the bug
> because on my machine, check_tsc_unstable() returns false.
> 
> I found out from Nadav that when KVM decides to run L2, it will write 
> vmcs01->tsc_offset + vmcs12->tsc_offset to the active TSC_OFFSET which seems right.
> But I verified that, if instead, I just write 
> vmcs01->tsc_offset to TSC_OFFSET in prepare_vmcs02(), I don't see the bug anymore.
> 
> Not sure where to go from here. I would appreciate if any one has any ideas.
> 
> 
> Bandan

Using guests TSC value when performing TSC adjustments is wrong. Can
you please try the following patch, which skips TSC adjustments if
vcpu is in guest mode.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2b76ae3..44c90d1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1096,6 +1096,9 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	s64 kernel_ns, max_kernel_ns;
 	u64 tsc_timestamp;
 
+	if (is_guest_mode(v))
+		return 0;
+
 	/* Keep irq disabled to prevent changes to the clock */
 	local_irq_save(flags);
 	kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
@@ -2214,6 +2217,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		tsc_delta = !vcpu->arch.last_guest_tsc ? 0 :
 			     tsc - vcpu->arch.last_guest_tsc;
 
+		if (is_guest_mode(vcpu))
+			tsc_delta = 0;
+
 		if (tsc_delta < 0)
 			mark_tsc_unstable("KVM discovered backwards TSC");
 		if (check_tsc_unstable()) {
@@ -2234,7 +2240,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
 	kvm_x86_ops->vcpu_put(vcpu);
 	kvm_put_guest_fpu(vcpu);
-	kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
+	if (!is_guest_mode(vcpu))
+		kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
 }
 
 static int is_efer_nx(void)
@@ -5717,7 +5724,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	if (hw_breakpoint_active())
 		hw_breakpoint_restore();
 
-	kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
+	if (!is_guest_mode(vcpu))
+		kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
 
 	vcpu->mode = OUTSIDE_GUEST_MODE;
 	smp_wmb();