linux-kernel - Re: [PATCH] KVM: x86: nVMX: allow RSM to restore VMXE CR4 flag

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Date:   Tue, 26 Mar 2019 17:02:27 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jon Doron <arilou@...il.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: nVMX: allow RSM to restore VMXE CR4 flag

> On 26 Mar 2019, at 15:48, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> Liran Alon <liran.alon@...cle.com> writes:
> 
>>> On 26 Mar 2019, at 15:07, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>> - Instread of putting the temporary HF_SMM_MASK drop to
>>> rsm_enter_protected_mode() (as was suggested by Liran), move it to
>>> emulator_set_cr() modifying its interface. emulate.c seems to be
>>> vcpu-specifics-free at this moment, we may want to keep it this way.
>>> - It seems that Hyper-V+UEFI on KVM is still broken, I'm observing sporadic
>>> hangs even with this patch. These hangs, however, seem to be unrelated to
>>> rsm.
>> 
>> Feel free to share details on these hangs ;)
>> 
> 
> You've asked for it)
> 
> The immediate issue I'm observing is some sort of a lockup which is easy
> to trigger with e.g. "-usb -device usb-tablet" on Qemu command line; it
> seems we get too many interrupts and combined with preemtion timer for
> L2 we're not making any progress:
> 
> kvm_userspace_exit:   reason KVM_EXIT_IOAPIC_EOI (26)
> kvm_set_irq:          gsi 18 level 1 source 0
> kvm_msi_set_irq:      dst 0 vec 177 (Fixed|physical|level)
> kvm_apic_accept_irq:  apicid 0 vec 177 (Fixed|edge)
> kvm_fpu:              load
> kvm_entry:            vcpu 0
> kvm_exit:             reason VMRESUME rip 0xfffff80000848115 info 0 0
> kvm_entry:            vcpu 0
> kvm_exit:             reason PREEMPTION_TIMER rip 0xfffff800f4448e01 info 0 0
> kvm_nested_vmexit:    rip fffff800f4448e01 reason PREEMPTION_TIMER info1 0 info2 0 int_info 0 int_info_err 0
> kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 800000b1 int_info_err 0
> kvm_entry:            vcpu 0
> kvm_exit:             reason APIC_ACCESS rip 0xfffff8000081fe11 info 10b0 0
> kvm_apic:             apic_write APIC_EOI = 0x0
> kvm_eoi:              apicid 0 vector 177
> kvm_fpu:              unload
> kvm_userspace_exit:   reason KVM_EXIT_IOAPIC_EOI (26)
> ...
> (and the pattern repeats)
> 
> Maybe it is a usb-only/Qemu-only problem, maybe not.
> 
> -- 
> Vitaly

The trace of kvm_apic_accept_irq should indicate that __apic_accept_irq() was called to inject an interrupt to L1 guest.
(I know that now we are running in L1 because next exit is a VMRESUME).

However, it is surprising to see that on next entry to guest, no interrupt was injected by vmx_inject_irq().
It may be because L1 guest is currently running with interrupt disabled and therefore only an IRQ-window was requested.
(Too bad we don’t have a trace for this…)

Next, we got an exit from L1 guest on VMRESUME. As part of it’s handling, active VMCS was changed from vmcs01 to vmcs02.
I believe the immediate exit later on preemption-timer was because the immediate-exit-request mechanism was invoked
which is now implemented by setting a VMX preemption-timer with value of 0 (Thanks to Sean).
(See vmx_vcpu_run() -> vmx_update_hv_timer() -> vmx_arm_hv_timer(vmx, 0)).
(Note that the pending interrupt was evaluated because of a recent patch of mine to nested_vmx_enter_non_root_mode()
to request KVM_REQ_EVENT when vmcs01 have requested an IRQ-window)

Therefore when entering L2, you immediately get an exit on PREEMPTION_TIMER which will cause eventually L0 to call
vmx_check_nested_events() which notices now the pending interrupt that should have been injected before to L1
and now exit from L2 to L1 on EXTERNAL_INTERRUPT on vector 0xb1.

Then L1 handles the interrupt by performing an EOI to LAPIC which propagate an EOI to IOAPIC which immediately re-inject
the interrupt (after clearing the remote_irr) as the irq-line is still set. i.e. QEMU’s ioapic_eoi_broadcast() calls ioapic_service() immediate after it clears remote-irr for this pin.

Also note that in trace we see only a single kvm_set_irq to level 1 but we don’t see immediately another kvm_set_irq to level 0.
This should indicate that in QEMU’s IOAPIC redirection-table, this pin is configured as level-triggered interrupt.
However, the trace of kvm_apic_accept_irq indicates that this interrupt is raised as an edge-triggered interrupt.

To sum up:
1) I would create a patch to add a trace to vcpu_enter_guest() when calling enable_smi_window() / enable_nmi_window() / enable_irq_window().
2) It is worth investigating why MSI trigger-mode is edge-triggered instead of level-triggered.
3) If this is indeed a level-triggered interrupt, it is worth investigating how the interrupt source behaves. i.e. What cause this device to lower the irq-line?
(As we don’t see any I/O Port or MMIO access by L1 guest interrupt-handler before performing the EOI)
4) Does this issue reproduce also when running with kernel-irqchip? (Instead of split-irqchip)

-Liran