Re: [PATCH Part2 v5 21/45] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

[Sean Christopherson]

On Fri, Aug 20, 2021, Brijesh Singh wrote:
> Implement a workaround for an SNP erratum where the CPU will incorrectly
> signal an RMP violation #PF if a hugepage (2mb or 1gb) collides with the
> RMP entry of a VMCB, VMSA or AVIC backing page.

...

> @@ -4539,6 +4539,16 @@ static int svm_vm_init(struct kvm *kvm)
>  	return 0;
>  }
>  
> +static void *svm_alloc_apic_backing_page(struct kvm_vcpu *vcpu)
> +{
> +	struct page *page = snp_safe_alloc_page(vcpu);
> +
> +	if (!page)
> +		return NULL;
> +
> +	return page_address(page);
> +}
> +
>  static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.hardware_unsetup = svm_hardware_teardown,
>  	.hardware_enable = svm_hardware_enable,
> @@ -4667,6 +4677,8 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.complete_emulated_msr = svm_complete_emulated_msr,
>  
>  	.vcpu_deliver_sipi_vector = svm_vcpu_deliver_sipi_vector,
> +
> +	.alloc_apic_backing_page = svm_alloc_apic_backing_page,

IMO, this should be guarded by a module param or X86_BUG_* to make it clear that
this is a bug and not working as intended.

And doesn't the APIC page need these shenanigans iff AVIC is enabled? (the module
param, not necessarily in the VM)

>  };
>  
>  static struct kvm_x86_init_ops svm_init_ops __initdata = {
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index d1f1512a4b47..e40800e9c998 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -575,6 +575,7 @@ void sev_es_create_vcpu(struct vcpu_svm *svm);
>  void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
>  void sev_es_prepare_guest_switch(struct vcpu_svm *svm, unsigned int cpu);
>  void sev_es_unmap_ghcb(struct vcpu_svm *svm);
> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu);
>  
>  /* vmenter.S */
>  
> -- 
> 2.17.1
>