Re: [RFC PATCH v1 05/37] KVM: guest_memfd: Wire up kvm_get_memory_attributes() to per-gmem attributes

[Alexey Kardashevskiy <aik@amd.com>]

On 29/1/26 12:16, Jason Gunthorpe wrote:
> On Wed, Jan 28, 2026 at 05:03:27PM -0800, Sean Christopherson wrote:
> 
>> For a dmabuf fd, the story is the same as guest_memfd.  Unless private vs. shared
>> is all or nothing, and can never change, then the only entity that can track that
>> info is the owner of the dmabuf.  And even if the private vs. shared attributes
>> are constant, tracking it external to KVM makes sense, because then the provider
>> can simply hardcode %true/%false.
> 
> Oh my I had not given that bit any thought. My remarks were just about
> normal non-CC systems.
> 
> So MMIO starts out shared, and then converts to private when the guest
> triggers it. It is not all or nothing, there are permanent shared
> holes in the MMIO ranges too.
> 
> Beyond that I don't know what people are thinking.
> 
> Clearly VFIO has to revoke and disable the DMABUF once any of it
> becomes private.

huh? Private MMIO still has to be mapped in the NPT (well, on AMD). It is the userspace mapping which we do not want^wneed and we do not by using dmabuf.

> VFIO will somehow have to know when it changes modes
> from the TSM subsystem.
> 
> I guess we could have a special channel for KVM to learn the
> shared/private page by page from VFIO as some kind of "aware of CC"
> importer.

Yilun is doing something like that in (there must be a newer version somewhere)
https://lore.kernel.org/all/20250529053513.1592088-1-yilun.xu@linux.intel.com/


> I suppose AMD needs to mangle the RMP when it changes, and KVM has to
> do that.

True.

> I forget what ARM does, but I seem to recall there is a call to create
> a vPCI function and that is what stuffs the S2? So maybe KVM isn't
> even involved? (IIRC people were talking that something else would
> call the vPCI function but I haven't seen patches)
> 
> No idea what x86 does beyond it has to unmap all the MMIO otherwise
> the machine crashes :P

When it is in the hypervisor area, there is no "x86" :)

The "AMD x86" does not crash if there are mappings which won't work, it faults/fences when these are accessed.
  
> Oh man, what a horrible mess to even contemplate. I'm going to bed.
>
> 
> Jason

-- 
Alexey