[ANNOUNCE] PUCK Agenda - 2025.04.02 - No Topic

[ANNOUNCE] PUCK Agenda - 2025.04.02 - No Topic

[Sean Christopherson]

No scheduled topic for tomorrow, but PUCK is back on this week.

Re: [ANNOUNCE] PUCK Agenda - 2025.04.02 - No Topic

[Ackerley Tng]

Would like to add an agenda item for 2025-04-30's PUCK meeting: KVM
memory attributes vs guest_memfd shareability.

guest_memfd tracks shareability to determine whether a page can be
faulted by the host into userspace.

pKVM does not use kvm->mem_attr_array for tracking private/shared status
of a page, and for Coco VMs like TDX, there seems to be duplicate
tracking of private/shared status in guest_memfd's shareability and in
KVM's memory attributes.

I would like to discuss a proposal for shared/private conversions to be
performed through a guest_memfd (not KVM) ioctl instead of using
KVM_SET_MEMORY_ATTRIBUTES, where Coco VMs using guest_memfd for both
shared and private memory can be able to (with some other changes around
KVM memory attributes) skip tracking private/shared in KVM's memory
attributes.


Thank you!

Re: [ANNOUNCE] PUCK Agenda - 2025.04.02 - No Topic

[Sean Christopherson]

On Tue, Apr 29, 2025, Ackerley Tng wrote:
> 
> Would like to add an agenda item for 2025-04-30's PUCK meeting: KVM
> memory attributes vs guest_memfd shareability.

Does next week work for you?  I.e. May 7th.  I won't be able to make tomorrow's
PUCK (about to send a cancelation mail).

> guest_memfd tracks shareability to determine whether a page can be
> faulted by the host into userspace.
> 
> pKVM does not use kvm->mem_attr_array for tracking private/shared status
> of a page, and for Coco VMs like TDX, there seems to be duplicate
> tracking of private/shared status in guest_memfd's shareability and in
> KVM's memory attributes.
> 
> I would like to discuss a proposal for shared/private conversions to be
> performed through a guest_memfd (not KVM) ioctl instead of using
> KVM_SET_MEMORY_ATTRIBUTES, where Coco VMs using guest_memfd for both
> shared and private memory can be able to (with some other changes around
> KVM memory attributes) skip tracking private/shared in KVM's memory
> attributes.

Has the proposal been posted on-list anywhere?  I haven't been following the
guest_memfd threads very closely (understatement).

Re: [ANNOUNCE] PUCK Agenda - 2025.04.02 - No Topic

[Ackerley Tng]

Sean Christopherson <seanjc@google.com> writes:

> On Tue, Apr 29, 2025, Ackerley Tng wrote:
>> 
>> Would like to add an agenda item for 2025-04-30's PUCK meeting: KVM
>> memory attributes vs guest_memfd shareability.
>
> Does next week work for you?  I.e. May 7th.  I won't be able to make tomorrow's
> PUCK (about to send a cancelation mail).
>
>> guest_memfd tracks shareability to determine whether a page can be
>> faulted by the host into userspace.
>> 
>> pKVM does not use kvm->mem_attr_array for tracking private/shared status
>> of a page, and for Coco VMs like TDX, there seems to be duplicate
>> tracking of private/shared status in guest_memfd's shareability and in
>> KVM's memory attributes.
>> 
>> I would like to discuss a proposal for shared/private conversions to be
>> performed through a guest_memfd (not KVM) ioctl instead of using
>> KVM_SET_MEMORY_ATTRIBUTES, where Coco VMs using guest_memfd for both
>> shared and private memory can be able to (with some other changes around
>> KVM memory attributes) skip tracking private/shared in KVM's memory
>> attributes.
>
> Has the proposal been posted on-list anywhere?  I haven't been following the
> guest_memfd threads very closely (understatement).

We managed to get it discussed at the guest_memfd upstream call. Here are
the slides, updated with discussion notes:
https://lpc.events/event/18/contributions/1764/attachments/1409/3708/2025-05-01-kvm-memory-attributes-vs-guest_memfd-shareability.pdf

Please remove the topic from the next PUCK! Thanks.