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

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Message-ID: <diqzv7k5emza.fsf@google.com>
Date: Thu, 23 Oct 2025 07:01:13 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
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Subject: Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2

Sean Christopherson <seanjc@...gle.com> writes:

> 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.

That's true.

> 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.
>

IIUC KVM_SET_MEMORY_ATTRIBUTES doesn't fail with -ENOTTY now when there
are no valid attributes.

Even if there's no valid attributes (as in
kvm_supported_mem_attributes() returns 0), it's possible to call
KVM_SET_MEMORY_ATTRIBUTES with .attributes set to 0, which will be a
no-op, but will return 0.

I think this is kind of correct behavior since .attributes = 0 is
actually a valid expression for "I want this range to be shared", and
for a VM that doesn't support private memory, it's a valid expression.


The other way that there are "no attributes" would be if there are no
/VM/ attributes, in which case KVM_SET_MEMORY_ATTRIBUTES, sent to as a
vm ioctl, will return -ENOTTY.

> 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

Did you mean KVM_CAP_MEMORY_ATTRIBUTES2 in the line above?

> 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.
>

Okay I'll switch this back to what it was.

>> @@ -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.
>

Will do.

>> +
>> +struct kvm_memory_attributes2 {
>> +	__u64 address;
>> +	__u64 size;
>> +	__u64 attributes;
>> +	__u64 flags;
>> +	__u64 reserved[4];
>
> Maybe be paranoid and reserve 12 u64s?

Will do.