Re: [PATCH v8 24/46] KVM: guest_memfd: Make in-place conversion the default

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 24, 2026, Ackerley Tng wrote:
> Yan Zhao <yan.y.zhao@intel.com> writes:
> > With gmem_in_place_conversion=true, userspace can create guest_memfd without the
> > MMAP flag. In such cases, shared memory is allocated from different backends.
> > This means this module parameter only enables per-gmem memory attribute and does
> > not guarantee that gmem in-place conversion will actually occur.

KVM module params are pretty much always about what KVM supports, not what is
guaranteed to happen.

  - enable_mmio_caching doesn't guarantee there will actually be MMIO SPTEs,
    because maybe the guest never accesses emulated MMIO.
  - enable_pmu doesn't guarantee VMs will get a PMU, because userspace may elect
    not to advertise one.
  - and so on and so forth...

Yes, there's a small mental jump to get from "KVM supports in-place conversion"
to "I need to set memory attributes on the guest_memfd instance, not the VM",
but I don't see that as a big hurdle, certainly not in the long term.  And once
the VMM code is written, I really do think most people are going to care about
whether or not KVM supports in-place conversion, not where PRIVATE is tracked.

> > To avoid confusion, could we rename this module parameter to something more
> > accurate, such as gmem_memory_attribute?
> 
> I asked Sean about this after getting some fixes off list. Sean said
> gmem_in_place_conversion is named for a host admin to use, and something
> like gmem_memory_attributes is too much implementation details for the
> admin.
> 
> Sean, would you reconsider since Yan also asked? If the admin compiled
> the kernel knowing what CONFIG_KVM_VM_MEMORY_ATTRIBUTES means, then the
> admin would also be able to use a param like gmem_memory_attributes?

No, because it's not all memory attributes, it's very specifically the PRIVATE
attribute that will get moved to guest_memfd.  I don't want to pick a name that
will become stale and confusing when RWX attributes come along.  The RWX bits
will be per-VM, while PRIVATE will be per-guest_memfd.