linux-kernel - Re: [PATCH v3 03/14] KVM: SVM: Detect X2APIC virtualization (x2AVIC) support

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Date:   Thu, 05 May 2022 10:38:17 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v3 03/14] KVM: SVM: Detect X2APIC virtualization
 (x2AVIC) support

On Thu, 2022-05-05 at 11:07 +0700, Suravee Suthikulpanit wrote:
> Maxim,
> 
> On 5/4/22 7:12 PM, Maxim Levitsky wrote:
> > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > > index a8f514212b87..fc3ba6071482 100644
> > > --- a/arch/x86/kvm/svm/avic.c
> > > +++ b/arch/x86/kvm/svm/avic.c
> > > @@ -40,6 +40,15 @@
> > >   #define AVIC_GATAG_TO_VMID(x)		((x >> AVIC_VCPU_ID_BITS) & AVIC_VM_ID_MASK)
> > >   #define AVIC_GATAG_TO_VCPUID(x)		(x & AVIC_VCPU_ID_MASK)
> > >   
> > > +enum avic_modes {
> > > +	AVIC_MODE_NONE = 0,
> > > +	AVIC_MODE_X1,
> > > +	AVIC_MODE_X2,
> > > +};
> > > +
> > > +static bool force_avic;
> > > +module_param_unsafe(force_avic, bool, 0444);
> > > +
> > >   /* Note:
> > >    * This hash table is used to map VM_ID to a struct kvm_svm,
> > >    * when handling AMD IOMMU GALOG notification to schedule in
> > > @@ -50,6 +59,7 @@ static DEFINE_HASHTABLE(svm_vm_data_hash, SVM_VM_DATA_HASH_BITS);
> > >   static u32 next_vm_id = 0;
> > >   static bool next_vm_id_wrapped = 0;
> > >   static DEFINE_SPINLOCK(svm_vm_data_hash_lock);
> > > +static enum avic_modes avic_mode;
> > >   
> > >   /*
> > >    * This is a wrapper of struct amd_iommu_ir_data.
> > > @@ -1077,3 +1087,33 @@ void avic_vcpu_unblocking(struct kvm_vcpu *vcpu)
> > >   
> > >   	avic_vcpu_load(vcpu);
> > >   }
> > > +
> > > +/*
> > > + * Note:
> > > + * - The module param avic enable both xAPIC and x2APIC mode.
> > > + * - Hypervisor can support both xAVIC and x2AVIC in the same guest.
> > > + * - The mode can be switched at run-time.
> > > + */
> > > +bool avic_hardware_setup(struct kvm_x86_ops *x86_ops)
> > > +{
> > > +	if (!npt_enabled)
> > > +		return false;
> > > +
> > > +	if (boot_cpu_has(X86_FEATURE_AVIC)) {
> > > +		avic_mode = AVIC_MODE_X1;
> > > +		pr_info("AVIC enabled\n");
> > > +	} else if (force_avic) {
> > > +		pr_warn("AVIC is not supported in CPUID but force enabled");
> > > +		pr_warn("Your system might crash and burn");
> > I think in this case avic_mode should also be set to AVIC_MODE_X1
> > (Hopefully this won't be needed for systems that have x2avic enabled)
> 
> You are right. My appology :(
> 
> Actually, x2AVIC depends on both CPUID bits (i.e. X86_FEATURE_AVIC and X86_FEATURE_X2AVIC).
> If the force_avic option is only applicable to only the X86_FEATURE_AVIC bit, we would
> need to check for the following condition before enabling x2AVIC support in the driver:
> 
>    if ((X86_FEATURE_AVIC | avic_force) & X86_FEATURE_X2AVIC)
> 	avic_mode = AVIC_MODE_X2

Let it be just for AVIC_MODE_X1, that is

else if (force_avic) {
	pr_warn("AVIC is not supported in CPUID but force enabled");
	pr_warn("Your system might crash and burn");
	avic_mode = AVIC_MODE_X1;

Best regards,
	Maxim Levitsky

> 
> Regards,
> Suravee
> 
>