On Thu, Jun 25, 2026 at 09:51:01AM +0800, Yan Zhao wrote:

On Wed, Jun 24, 2026 at 05:41:58PM -0700, Sean Christopherson wrote:

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On Wed, Jun 24, 2026, Ackerley Tng wrote:

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Yan Zhao [off-list ref] writes:

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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.

Sorry, I just saw this mail after posting my reply in [1].

I'm ok with gmem_in_place_conversion=true just means KVM supports in-place
conversion, while we can still create VMs with shared memory not from gmem.

Or what about "allow_gmem_in_place_conversion" ?

Though it still feels a bit odd to require TDX huge pages to depend on
gmem_in_place_conversion=true when shared memory is not currently allocated from
gmem, it should become more natural over time once gmem supports in-place
conversions for huge page.

[1] https://lore.kernel.org/all/ajyCn0PnFtQK+Nka@yzhao56-desk.sh.intel.com (local)

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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.