Re: Nested VMX - L1 hangs on running L2

Marcelo Tosatti
2011-07-18T18:26:53Z

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();