[PATCH] kvm: x86: Update KVM_GET_CPUID2 to return valid entry count

[PATCH] kvm: x86: Update KVM_GET_CPUID2 to return valid entry count

[Takahiro Itazuri]

Modify the KVM_GET_CPUID2 API to return the number of valid entries in
nent field of kvm_cpuid2, even when the API is successful.

Previously, the KVM_GET_CPUID2 API only updated the nent field when an
error was returned. If the API was called with an entry count larger
than necessary (e.g., KVM_MAX_CPUID_ENTRIES), it would succeed, but the
nent field would continue to show a value larger than the actual number
of entries filled by the KVM_GET_CPUID2 API. With this change, users can
rely on the updated nent field and there is no need to traverse
unnecessary entries and check whether an entry is valid or not.

Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
---
 arch/x86/kvm/cpuid.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 599aebec2d52..31838dfddda6 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -523,10 +523,13 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
 			      struct kvm_cpuid2 *cpuid,
 			      struct kvm_cpuid_entry2 __user *entries)
 {
-	int r;
+	int nent, r;
+
+	nent = cpuid->nent;
+	cpuid->nent = vcpu->arch.cpuid_nent;
 
 	r = -E2BIG;
-	if (cpuid->nent < vcpu->arch.cpuid_nent)
+	if (nent < vcpu->arch.cpuid_nent)
 		goto out;
 	r = -EFAULT;
 	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
@@ -535,7 +538,6 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
 	return 0;
 
 out:
-	cpuid->nent = vcpu->arch.cpuid_nent;
 	return r;
 }
 
-- 
2.39.2

Re: [PATCH] kvm: x86: Update KVM_GET_CPUID2 to return valid entry count

[Sean Christopherson]

Capitalize KVM please, i.e. "KVM: x86:".

On Mon, Apr 10, 2023, Takahiro Itazuri wrote:
> Modify the KVM_GET_CPUID2 API to return the number of valid entries in
> nent field of kvm_cpuid2, even when the API is successful.
> 
> Previously, the KVM_GET_CPUID2 API only updated the nent field when an

Heh, I am so used to KVM_SET_CPUID2 being a source of bugs that it took me at
least three read throughs before I caught that this is fixing the GET side.

> error was returned. If the API was called with an entry count larger
> than necessary (e.g., KVM_MAX_CPUID_ENTRIES), it would succeed, but the
> nent field would continue to show a value larger than the actual number
> of entries filled by the KVM_GET_CPUID2 API. With this change, users can
> rely on the updated nent field and there is no need to traverse
> unnecessary entries and check whether an entry is valid or not.

While I completely agree that the not updating "nent" on success is asinine, I
am mildly concerned about this being a breaking ABI change, e.g. if a VMM has
"nent" marked as a consant/immutable value.  I suspect it's ok because AFAICT,
pretty much nothing outside of selftests actually uses KVM_GET_CPUID2.  But at
the very least, I'll push this out until 6.5 so that it can soak in linux-next
for a long time.

Paolo, any thoughts/objections?

> Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
> ---
>  arch/x86/kvm/cpuid.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 599aebec2d52..31838dfddda6 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -523,10 +523,13 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>  			      struct kvm_cpuid2 *cpuid,
>  			      struct kvm_cpuid_entry2 __user *entries)
>  {
> -	int r;
> +	int nent, r;
> +
> +	nent = cpuid->nent;
> +	cpuid->nent = vcpu->arch.cpuid_nent;
>  
>  	r = -E2BIG;
> -	if (cpuid->nent < vcpu->arch.cpuid_nent)
> +	if (nent < vcpu->arch.cpuid_nent)
>  		goto out;
>  	r = -EFAULT;
>  	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
> @@ -535,7 +538,6 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>  	return 0;
>  
>  out:
> -	cpuid->nent = vcpu->arch.cpuid_nent;
>  	return r;

I think we should break from the (IMO) somewhat funky KVM ioctl() pattern of

	r = <errno>
	if (try something and it fails)
		goto out;

and instead set "r" in the error paths.  That avoids the need for a scratch "nent",
and IMO makes this much more straightforward.

	int r = 0;

	if (cpuid->nent < vcpu->arch.cpuid_nent)
		r = -E2BIG;
	else if (copy_to_user(entries, vcpu->arch.cpuid_entries,
			      vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
		r = -EFAULT;

	/*
	 * Update "nent" even on failure, e.g. so that userspace can fix an
	 * -E2BIG issue by allocating a larger array.
	 */
	cpuid->nent = vcpu->arch.cpuid_nent;
	return r;

Re: [PATCH] kvm: x86: Update KVM_GET_CPUID2 to return valid entry count

[Takahiro Itazuri]

Date:   Mon, 10 Apr 2023 08:47:05 -0700
From:   Sean Christopherson <seanjc@google.com>
> Capitalize KVM please, i.e. "KVM: x86:".

Will fix. Thanks!

> I think we should break from the (IMO) somewhat funky KVM ioctl() pattern of
> 
> 	r = <errno>
> 	if (try something and it fails)
> 		goto out;
> 
> and instead set "r" in the error paths.  That avoids the need for a scratch "nent",
> and IMO makes this much more straightforward.
> 
> 	int r = 0;
> 
> 	if (cpuid->nent < vcpu->arch.cpuid_nent)
> 		r = -E2BIG;
> 	else if (copy_to_user(entries, vcpu->arch.cpuid_entries,
> 			      vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
> 		r = -EFAULT;
> 
> 	/*
> 	 * Update "nent" even on failure, e.g. so that userspace can fix an
> 	 * -E2BIG issue by allocating a larger array.
> 	 */
> 	cpuid->nent = vcpu->arch.cpuid_nent;
> 	return r;

Looks better to me! Will fix this too!.

Best regards,
Takahiro Itazuri