Message-ID: <YQRkBI9RFf6lbifZ@google.com>
Date:   Fri, 30 Jul 2021 20:41:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tao Xu <tao3.xu@...el.com>
Cc:     pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On Tue, May 25, 2021, Tao Xu wrote:
>  #endif /* __KVM_X86_VMX_CAPS_H */
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4bceb5ca3a89..c0ad01c88dac 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -205,6 +205,10 @@ module_param(ple_window_max, uint, 0444);
>  int __read_mostly pt_mode = PT_MODE_SYSTEM;
>  module_param(pt_mode, int, S_IRUGO);
>  
> +/* Default is 0, less than 0 (for example, -1) disables notify window. */
> +static int __read_mostly notify_window;

I'm not sure I like the idea of trusting ucode to select an appropriate internal
threshold.  Unless the internal threshold is architecturally defined to be at
least N nanoseconds or whatever, I think KVM should provide its own sane default.
E.g. it's not hard to imagine a scenario where a ucode patch gets rolled out that
adjusts the threshold and starts silently degrading guest performance.

Even if the internal threshold isn't architecturally constrained, it would be very,
very helpful if Intel could publish the per-uarch/stepping thresholds, e.g. to give
us a ballpark idea of how agressive KVM can be before it risks false positives.

> +module_param(notify_window, int, 0644);

I really like the idea of making the module param writable, but doing so will
require far more effort.  At an absolute minimum, the module param would need to
be snapshotted at VM creation time, a la lapic_timer_advance_ns, otherwise the
behavior is non-deterministic.

But I don't think snapshotting is a worthwhile approach because the main reason
for adjusting the window while guests are running is probably going to be to relax
the window because guest's are observing degraded performance.  Hopefully that
never happens, but the "CPU adds a magic internal buffer" behavior makes me more
than a bit nervous.

And on the other hand, adding a ton of logic to forcefully update every VMCS is
likely overkill.

So, that takes us back to providing a sane, somewhat conservative default.  I've
said in the past that ideally the notify_window would be as small as possible,
but pushing it down to single digit cycles swings the pendulum too far in the
other direction.

> +
>  static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
>  static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
>  static DEFINE_MUTEX(vmx_l1d_flush_mutex);
> @@ -2539,7 +2543,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  			SECONDARY_EXEC_PT_USE_GPA |
>  			SECONDARY_EXEC_PT_CONCEAL_VMX |
>  			SECONDARY_EXEC_ENABLE_VMFUNC |
> -			SECONDARY_EXEC_BUS_LOCK_DETECTION;
> +			SECONDARY_EXEC_BUS_LOCK_DETECTION |
> +			SECONDARY_EXEC_NOTIFY_VM_EXITING;
>  		if (cpu_has_sgx())
>  			opt2 |= SECONDARY_EXEC_ENCLS_EXITING;
>  		if (adjust_vmx_controls(min2, opt2,
> @@ -4376,6 +4381,9 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
>  	if (!vcpu->kvm->arch.bus_lock_detection_enabled)
>  		exec_control &= ~SECONDARY_EXEC_BUS_LOCK_DETECTION;
>  
> +	if (cpu_has_notify_vm_exiting() && notify_window < 0)
> +		exec_control &= ~SECONDARY_EXEC_NOTIFY_VM_EXITING;
> +
>  	vmx->secondary_exec_control = exec_control;
>  }
>  
> @@ -4423,6 +4431,9 @@ static void init_vmcs(struct vcpu_vmx *vmx)
>  		vmx->ple_window_dirty = true;
>  	}
>  
> +	if (cpu_has_notify_vm_exiting() && notify_window >= 0)
> +		vmcs_write32(NOTIFY_WINDOW, notify_window);

I'm all for punting full nested support to a future patch, but _this_ patch
absolutely needs to apply KVM's notify_window to vmcs02, otherwise L1 can simply
run in L2 to avoid the restriction.  init_vmcs() is used only for vmcs01, i.e.
prepare_vmcs02_constant_state() needs to set the correct vmcs.NOTIFY_WINDOW,
and prepare_vmcs02_early() needs to set/clear SECONDARY_EXEC_NOTIFY_VM_EXITING
appropriately.

> +
>  	vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, 0);
>  	vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, 0);
>  	vmcs_write32(CR3_TARGET_COUNT, 0);           /* 22.2.1 */
> @@ -5642,6 +5653,31 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static int handle_notify(struct kvm_vcpu *vcpu)
> +{
> +	unsigned long exit_qual = vmx_get_exit_qual(vcpu);
> +
> +	if (!(exit_qual & NOTIFY_VM_CONTEXT_INVALID)) {

What does CONTEXT_INVALID mean?  The ISE doesn't provide any information whatsoever.

> +		/*
> +		 * Notify VM exit happened while executing iret from NMI,
> +		 * "blocked by NMI" bit has to be set before next VM entry.
> +		 */
> +		if (enable_vnmi &&
> +		    (exit_qual & INTR_INFO_UNBLOCK_NMI))
> +			vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
> +				      GUEST_INTR_STATE_NMI);

Hmm, logging of some kind is probably a good idea if this exit occurs, e.g. so
that the host can (a) get an indication that a guest is potentially malicious and
(b) rule out (or confirm) notify_window exits as the source of degraded guest
performance.

Maybe add a per-vCPU stat, "u64 notify_window_exits"?

Another thought would be to also do pr_info/warn_ratelimited if a vCPU gets
multiple notify_window exits and doesn't appear to be making forward progress,
e.g. same RIP observed two notify_window exits in a row.  Even if the guest is
making forward progress, displaying the guest RIP and instruction (if possible)
could be useful in triaging why the guest appears to be getting false positives.

> +		return 1;
> +	}
> +
> +	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> +	vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_NO_EVENT_WINDOW;
> +	vcpu->run->internal.ndata = 1;
> +	vcpu->run->internal.data[0] = exit_qual;

Unless an invalid context can _never_ happen, or is already fatal to the guest,
I don't think effectively killing the guest is a good idea.  KVM doesn't know
for certain that the guest was being malicious, all it knows is that the CPU
didn't open an event window for some arbitrary amount of time (arbitrary because
the internal threshold is likely to be uarch specific).  KVM is getting exits,
which means it's getting a chance to check for signals, etc..., so resuming the
guest is ok.

> +
> +	return 0;
> +}
> +
>  /*
>   * The exit handlers return 1 if the exit was handled fully and guest execution
>   * may resume.  Otherwise they set the kvm_run parameter to indicate what needs
> @@ -5699,6 +5735,7 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>  	[EXIT_REASON_PREEMPTION_TIMER]	      = handle_preemption_timer,
>  	[EXIT_REASON_ENCLS]		      = handle_encls,
>  	[EXIT_REASON_BUS_LOCK]                = handle_bus_lock_vmexit,
> +	[EXIT_REASON_NOTIFY]		      = handle_notify,
>  };
>  

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