linux-kernel - Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

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Date:   Tue, 3 Aug 2021 08:38:13 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Tao Xu <tao3.xu@...el.com>, 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
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On 8/2/2021 11:46 PM, Sean Christopherson wrote:
> On Mon, Aug 02, 2021, Xiaoyao Li wrote:
>> On 7/31/2021 4:41 AM, Sean Christopherson wrote:
>>> 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.
>>
>> You mean when internal threshold gets smaller somehow, and cases
>> false-positive that leads unexpected VM exit on normal instruction? In this
>> case, we set increase the vmcs.notify_window in KVM.
> 
> Not while VMs are running though.
> 
>> I think there is no better to avoid this case if ucode changes internal
>> threshold. Unless KVM's default notify_window is bigger enough.
>>
>>> 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.
>>
>> Even Intel publishes the internal threshold, we still need to provide a
>> final best_value (internal + vmcs.notify_window). Then what's that value?
> 
> The ideal value would be high enough to guarantee there are zero false positives,
> yet low enough to prevent a malicious guest from causing instability in the host
> by blocking events for an extended duration.  The problem is that there's no
> magic answer for the threshold at which a blocked event would lead to system
> instability, and without at least a general idea of the internal value there's no
> answer at all.
> 
> IIRC, SGX instructions have a hard upper bound of 25k cycles before they have to
> check for pending interrupts, e.g. it's why EINIT is interruptible.  The 25k cycle
> limit is likely a good starting point for the combined minimum.  That's why I want
> to know the internal minimum; if the internal minimum is _guaranteed_ to be >25k,
> then KVM can be more aggressive with its default value.

OK. I will go internally to see if we can publish the internal threshold.

>> If we have an option for final best_value, then I think it's OK to just let
>> vmcs.notify_window = best_value. Then the true final value is best_value +
>> internal.
>>   - if it's a normal instruction, it should finish within best_value or
>> best_value + internal. So it makes no difference.
>>   - if it's an instruction in malicious case, it won't go to next instruction
>> whether wait for best_value or best_value + internal.
> 
> ...
> 
>>>> +
>>>>    	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.
>>
>> It means whether the VM context is corrupted and not valid in the VMCS.
> 
> Well that's a bit terrifying.  Under what conditions can the VM context become
> corrupted?  E.g. if the context can be corrupted by an inopportune NOTIFY exit,
> then KVM needs to be ultra conservative as a false positive could be fatal to a
> guest.
> 

Short answer is no case will set the VM_CONTEXT_INVALID bit.

VM_CONTEXT_INVALID is so fatal and IMHO it won't be set for any 
inopportune NOTIFY exit.