linux-kernel - Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY

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Message-ID: <aPlpKbHGea90IebS@google.com>
Date: Wed, 22 Oct 2025 16:30:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2

On Wed, Oct 22, 2025, Ackerley Tng wrote:
> Ackerley Tng <ackerleytng@...gle.com> writes:
> 
> Found another issue with KVM_CAP_MEMORY_ATTRIBUTES2.
> 
> KVM_CAP_MEMORY_ATTRIBUTES2 was defined to do the same thing as
> KVM_CAP_MEMORY_ATTRIBUTES, but that's wrong since
> KVM_CAP_MEMORY_ATTRIBUTES2 should indicate the presence of
> KVM_SET_MEMORY_ATTRIBUTES2 and struct kvm_memory_attributes2.

No?  If no attributes are supported, whether or not KVM_SET_MEMORY_ATTRIBUTES2
exists is largely irrelevant.  We can even provide the same -ENOTTY errno by
checking that _any_ attributes are supported, i.e. so that doing
KVM_SET_MEMORY_ATTRIBUTES2 on KVM without any support whatsoever fails in the
same way that KVM with code support but no attributes fails.

In other words, I don't see why it can't do both.  Even if we can't massage the
right errno, I would much rather KVM_SET_MEMORY_ATTRIBUTES2 enumerate the set of
supported attributes than simply '1'.  E.g. we have no plans to support
KVM_SET_MEMORY_ATTRIBUTES on guest_memfd, and so returning simply '1' creates an
unwanted and unnecessary dependency.

> @@ -1617,4 +1618,15 @@ struct kvm_pre_fault_memory {
>  	__u64 padding[5];
>  };
>  
> +/* Available with KVM_CAP_MEMORY_ATTRIBUTES2 */
> +#define KVM_SET_MEMORY_ATTRIBUTES2              _IOWR(KVMIO,  0xd6, struct kvm_memory_attributes2)

Please use the same literal number, 0xd2, as

  #define KVM_SET_MEMORY_ATTRIBUTES              _IOW(KVMIO,  0xd2, struct kvm_memory_attributes)

The "final" ioctl number that userspace sees incorporates the directionality and
the size of the struct, i.e. KVM_SET_MEMORY_ATTRIBUTES and KVM_SET_MEMORY_ATTRIBUTES2
are guaranteed to be distinct even if they both use 0xd2 as the "minor" number.

> +
> +struct kvm_memory_attributes2 {
> +	__u64 address;
> +	__u64 size;
> +	__u64 attributes;
> +	__u64 flags;
> +	__u64 reserved[4];

Maybe be paranoid and reserve 12 u64s?