Re: [PATCH RFC v7 03/64] KVM: SVM: Advertise private memory support to KVM

[Michael Roth]

On Fri, Dec 23, 2022 at 05:56:50PM +0100, Borislav Petkov wrote:
> On Wed, Dec 14, 2022 at 01:39:55PM -0600, Michael Roth wrote:
> > +       bool (*private_mem_enabled)(struct kvm *kvm);
> 
> This looks like a function returning boolean to me. IOW, you can
> simplify this to:

The semantics and existing uses of KVM_X86_OP_OPTIONAL_RET0() gave me the
impression it needed to return an integer value, since by default if a
platform doesn't implement the op it would "return 0", and so could
still be called unconditionally.

Maybe that's not actually enforced, by it seems awkward to try to use a
bool return instead. At least for KVM_X86_OP_OPTIONAL_RET0().

However, we could just use KVM_X86_OP() to declare it so we can cleanly
use a function that returns bool, and then we just need to do:

  bool kvm_arch_has_private_mem(struct kvm *kvm)
  {
          if (kvm_x86_ops.private_mem_enabled)
                  return static_call(kvm_x86_private_mem_enabled)(kvm);
  }
    
instead of relying on default return value. So I'll take that approach
and adopt your other suggested changes.

...

On a separate topic though, at a high level, this hook is basically a way
for platform-specific code to tell generic KVM code that private memslots
are supported by overriding the kvm_arch_has_private_mem() weak
reference. In this case the AMD platform is using using kvm->arch.upm_mode
flag to convey that, which is in turn set by the
KVM_CAP_UNMAPPED_PRIVATE_MEMORY introduced in this series.

But if, as I suggested in response to your PATCH 2 comments, we drop
KVM_CAP_UNAMMPED_PRIVATE_MEMORY in favor of
KVM_SET_SUPPORTED_MEMORY_ATTRIBUTES ioctl to enable "UPM mode" in SEV/SNP
code, then we need to rethink things a bit, since KVM_SET_MEMORY_ATTRIBUTES
in-part relies on kvm_arch_has_private_mem() to determine what flags are
supported, whereas SEV/SNP code would be using what was set by
KVM_SET_MEMORY_ATTRIBUTES to determine the return value in
kvm_arch_has_private_mem().

So, for AMD, the return value of kvm_arch_has_private_mem() needs to rely
on something else. Maybe the logic can just be:

  bool svm_private_mem_enabled(struct kvm *kvm)
  {
    return sev_enabled(kvm) || sev_snp_enabled(kvm)
  }

(at least in the context of this patchset where UPM support is added for
both SEV and SNP).

So I'll plan to make that change as well.

-Mike

> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index 82ba4a564e58..4449aeff0dff 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -129,6 +129,7 @@ KVM_X86_OP(msr_filter_changed)
>  KVM_X86_OP(complete_emulated_msr)
>  KVM_X86_OP(vcpu_deliver_sipi_vector)
>  KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
> +KVM_X86_OP_OPTIONAL_RET0(private_mem_enabled);
>  
>  #undef KVM_X86_OP
>  #undef KVM_X86_OP_OPTIONAL
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1da0474edb2d..1b4b89ddeb55 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1574,6 +1574,7 @@ struct kvm_x86_ops {
>  
>  	void (*load_mmu_pgd)(struct kvm_vcpu *vcpu, hpa_t root_hpa,
>  			     int root_level);
> +	bool (*private_mem_enabled)(struct kvm *kvm);
>  
>  	bool (*has_wbinvd_exit)(void);
>  
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index ce362e88a567..73b780fa4653 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4680,6 +4680,14 @@ static int svm_vm_init(struct kvm *kvm)
>  	return 0;
>  }
>  
> +static bool svm_private_mem_enabled(struct kvm *kvm)
> +{
> +	if (sev_guest(kvm))
> +		return kvm->arch.upm_mode;
> +
> +	return IS_ENABLED(CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING);
> +}
> +
>  static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.name = "kvm_amd",
>  
> @@ -4760,6 +4768,8 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  
>  	.vcpu_after_set_cpuid = svm_vcpu_after_set_cpuid,
>  
> +	.private_mem_enabled = svm_private_mem_enabled,
> +
>  	.has_wbinvd_exit = svm_has_wbinvd_exit,
>  
>  	.get_l2_tsc_offset = svm_get_l2_tsc_offset,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 823646d601db..9a1ca59d36a4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12556,6 +12556,11 @@ void __user * __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa,
>  }
>  EXPORT_SYMBOL_GPL(__x86_set_memory_region);
>  
> +bool kvm_arch_has_private_mem(struct kvm *kvm)
> +{
> +	return static_call(kvm_x86_private_mem_enabled)(kvm);
> +}
> +
>  void kvm_arch_pre_destroy_vm(struct kvm *kvm)
>  {
>  	kvm_mmu_pre_destroy_vm(kvm);
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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