Re: [PATCH v2 4/4] x86: KVM: SVM: workaround for AVIC's errata #1235

[Sean Christopherson]

 On Thu, Sep 28, 2023, Maxim Levitsky wrote:
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 4b74ea91f4e6bb6..28bb0e6b321660d 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -62,6 +62,9 @@ static_assert(__AVIC_GATAG(AVIC_VM_ID_MASK, AVIC_VCPU_ID_MASK) == -1u);
>  static bool force_avic;
>  module_param_unsafe(force_avic, bool, 0444);
>  
> +static int avic_zen2_errata_workaround = -1;
> +module_param(avic_zen2_errata_workaround, int, 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
> @@ -276,7 +279,7 @@ static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
>  
>  static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  {
> -	u64 *entry, new_entry;
> +	u64 *entry;
>  	int id = vcpu->vcpu_id;
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> @@ -308,10 +311,10 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  	if (!entry)
>  		return -EINVAL;
>  
> -	new_entry = __sme_set((page_to_phys(svm->avic_backing_page) &
> -			      AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK) |
> -			      AVIC_PHYSICAL_ID_ENTRY_VALID_MASK);
> -	WRITE_ONCE(*entry, new_entry);
> +	svm->avic_physical_id_entry = __sme_set((page_to_phys(svm->avic_backing_page) &
> +						 AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK) |
> +						 AVIC_PHYSICAL_ID_ENTRY_VALID_MASK);
> +	WRITE_ONCE(*entry, svm->avic_physical_id_entry);

Aha!  Rather than deal with the dummy entry at runtime, simply point the pointer
at the dummy entry during setup.

And instead of adding a dedicated erratum param, let's piggyback VMX's enable_ipiv.
It's not a true disable, but IMO it's close enough.  That will make the param
much more self-documenting, and won't feel so awkward if someone wants to disable
IPI virtualization for other reasons.

Then we can do this in three steps:

  1. Move enable_ipiv to common code
  2. Let userspace disable enable_ipiv for SVM+AVIC
  3. Disable enable_ipiv for affected CPUs

The biggest downside to using enable_ipiv is that a the "auto" behavior for the
erratum will be a bit ugly, but that's a solvable problem.

If you've no objection to the above approach, I'll post the attached patches along
with a massaged version of this patch.

The attached patches apply on top of an AVIC clean[*], which (shameless plug)
could use a review ;-)

[*] https://lore.kernel.org/all/20230815213533.548732-1-seanjc@xxxxxxxxxx