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Message-ID: <diqzqzuse58c.fsf@google.com>
Date: Fri, 24 Oct 2025 07:36:51 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Steven Price <steven.price@....com>, cgroups@...r.kernel.org, kvm@...r.kernel.org,
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Subject: Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2
Sean Christopherson <seanjc@...gle.com> writes:
> On Thu, Oct 23, 2025, Ackerley Tng wrote:
>> 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.
>
> Ya, this is what I was trying to say with "_any_ attributes are supported". I.e.
> by "any" I meant "any attributes in KVM for VMs vs. gmems", not "any attributes
> for this specific VM/gmem instance".
>
>>
>> [...snip...]
>>
I've been thinking more about this:
#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
case KVM_CAP_MEMORY_ATTRIBUTES2:
case KVM_CAP_MEMORY_ATTRIBUTES:
if (!vm_memory_attributes)
return 0;
return kvm_supported_mem_attributes(kvm);
#endif
And the purpose of adding KVM_CAP_MEMORY_ATTRIBUTES2 is that
KVM_CAP_MEMORY_ATTRIBUTES2 tells userspace that
KVM_SET_MEMORY_ATTRIBUTES2 is available iff there are valid
attributes.
(So there's still a purpose)
Without valid attributes, userspace can't tell if it should use
KVM_SET_MEMORY_ATTRIBUTES or the 2 version.
I also added KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES, which tells
userspace the valid attributes when calling KVM_SET_MEMORY_ATTRIBUTES2
on a guest_memfd:
#ifdef CONFIG_KVM_GUEST_MEMFD
case KVM_CAP_GUEST_MEMFD:
return 1;
case KVM_CAP_GUEST_MEMFD_FLAGS:
return kvm_gmem_get_supported_flags(kvm);
case KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES:
if (vm_memory_attributes)
return 0;
return kvm_supported_mem_attributes(kvm);
#endif
So to set memory attributes, userspace should
if (kvm_check_cap(KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES) > 0)
use KVM_SET_MEMORY_ATTRIBUTES2 with guest_memfd
else if (kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES2) > 0)
use KVM_SET_MEMORY_ATTRIBUTES2 with VM fd
else if (kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES) > 0)
use KVM_SET_MEMORY_ATTRIBUTES with VM fd
else
can't set memory attributes
Something like that?
In selftests there's this, when KVM_SET_USER_MEMORY_REGION2 was
introduced:
#define TEST_REQUIRE_SET_USER_MEMORY_REGION2() \
__TEST_REQUIRE(kvm_has_cap(KVM_CAP_USER_MEMORY2), \
"KVM selftests now require KVM_SET_USER_MEMORY_REGION2 (introduced in v6.8)")
But looks like there's no direct equivalent for the introduction of
KVM_SET_MEMORY_ATTRIBUTES2?
The closest would be to add a TEST_REQUIRE_VALID_ATTRIBUTES() which
checks KVM_CAP_MEMORY_ATTRIBUTES2 or
KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES before making the vm or
guest_memfd ioctl respsectively.
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