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

[Yan Zhao]

On Wed, Jun 24, 2026 at 05:41:58PM -0700, Sean Christopherson wrote:
> On Wed, Jun 24, 2026, Ackerley Tng wrote:
> > Yan Zhao <yan.y.zhao@xxxxxxxxx> 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.
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.

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@xxxxxxxxxxxxxxxxxxxxxxxxx


> > > 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.