linux-kernel - Re: [PATCH v6 3/3] KVM: VMX: Enable Notify VM exit

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Date:   Thu, 19 May 2022 18:38:16 +0800
From:   Chenyi Qiang <chenyi.qiang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/3] KVM: VMX: Enable Notify VM exit



On 5/19/2022 6:30 AM, Sean Christopherson wrote:
> On Thu, Apr 21, 2022, Chenyi Qiang wrote:
>> @@ -1504,6 +1511,8 @@ struct kvm_x86_ops {
>>   	 * Returns vCPU specific APICv inhibit reasons
>>   	 */
>>   	unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
>> +
>> +	bool has_notify_vmexit;
> 
> I'm pretty sure I suggested this, but seeing it in code, it kinda sorta makes things
> worst if we don't first consolidate the existing flags.  kvm_x86_ops works, but we'd
> definitely be taking liberties with the "ops" part.
> 
> What about adding struct kvm_caps to collect these flags/settings that don't fit
> into kvm_cpu_caps because they're not a CPUID feature flag?  kvm_x86_ops has the
> advantage of kinda being read-only after init since VMX modifies vmx_x86_ops,
> but IMO that's not enough reason to shove this into kvm_x86_ops.  And long term,
> we might be able find a way to mark kvm_caps as full __ro_after_init.
> 
> If no one objects, the attached patch can slide in before this patch, then
> has_notifiy_vmexit can land in kvm_caps.
> 
> struct kvm_caps {
> 	/* control of guest tsc rate supported? */
> 	bool has_tsc_control;
> 	/* maximum supported tsc_khz for guests */
> 	u32  max_guest_tsc_khz;
> 	/* number of bits of the fractional part of the TSC scaling ratio */
> 	u8   tsc_scaling_ratio_frac_bits;
> 	/* maximum allowed value of TSC scaling ratio */
> 	u64  max_tsc_scaling_ratio;
> 	/* 1ull << kvm_caps.tsc_scaling_ratio_frac_bits */
> 	u64  default_tsc_scaling_ratio;
> 	/* bus lock detection supported? */
> 	bool has_bus_lock_exit;
> 
> 	u64 supported_mce_cap;
> 	u64 supported_xcr0;
> 	u64 supported_xss;
> };
> 

Thanks Sean for your patch. I think an unintentional change is mixed in it:

@@ -4739,7 +4725,8 @@ static int 
kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
  	return (kvm_arch_interrupt_allowed(vcpu) &&
  		kvm_cpu_accept_dm_intr(vcpu) &&
  		!kvm_event_needs_reinjection(vcpu) &&
-		!vcpu->arch.exception.pending);
+		!vcpu->arch.exception.pending &&
+		!kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu));
  }

Maybe this should belong to the patch 1?

>> @@ -6090,6 +6094,18 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>   		}
>>   		mutex_unlock(&kvm->lock);
>>   		break;
>> +	case KVM_CAP_X86_NOTIFY_VMEXIT:
>> +		r = -EINVAL;
>> +		if ((u32)cap->args[0] & ~KVM_X86_NOTIFY_VMEXIT_VALID_BITS)
>> +			break;
>> +		if (!kvm_x86_ops.has_notify_vmexit)
>> +			break;
>> +		if (!(u32)cap->args[0] & KVM_X86_NOTIFY_VMEXIT_ENABLED)
>> +			break;
>> +		kvm->arch.notify_window = cap->args[0] >> 32;
> 
> Setting notify_vmexit and notify_vmexit_flags needs to be done under kvm->lock,
> and changing notify_window if kvm->created_vcpus > 0 needs to disallowed, otherwise
> init_vmcs() will use the wrong value.
> 
> notify_vmexit_flags could be changed on the fly, but I doubt that's worth
> supporting as even the smallest amount of complexity will go unused.
> 
> So I think this?
> 

Make sense.

> 	case KVM_CAP_X86_NOTIFY_VMEXIT:
> 		r = -EINVAL;
> 		if ((u32)cap->args[0] & ~KVM_X86_NOTIFY_VMEXIT_VALID_BITS)
> 			break;
> 		if (!kvm_x86_ops.has_notify_vmexit)
> 			break;
> 		if (!(u32)cap->args[0] & KVM_X86_NOTIFY_VMEXIT_ENABLED)
> 			break;
> 		mutex_lock(&kvm->lock);
> 		if (!kvm->created_vcpus) {
> 			kvm->arch.notify_window = cap->args[0] >> 32;
> 			kvm->arch.notify_vmexit_flags = (u32)cap->args[0];
> 			r = 0;
> 		}
> 		mutex_unlock(&kvm->lock);
> 		break;