Re: [PATCH v3 08/11] KVM: arm64: Handle guest_memfd()-backed guest page faults

[Quentin Perret <qperret@google.com>]

On Tuesday 11 Feb 2025 at 16:34:02 (+0000), Fuad Tabba wrote:
> > Sorry, yes, that wasn't clear. I meant that kvm_mem_is_private() calls
> > kvm_get_memory_attributes() which indexes kvm->mem_attr_array. The
> > comment in struct kvm indicates that this xarray is protected by RCU for
> > readers, so I was just checking if we were relying on
> > kvm_handle_guest_abort() to take srcu_read_lock(&kvm->srcu) for us, or
> > if there was something else more subtle here.
> 
> I was kind of afraid that people would be confused by this, and I
> commented on it in the commit message of the earlier patch:
> https://lore.kernel.org/all/20250211121128.703390-6-tabba@google.com/
> 
> > Note that the word "private" in the name of the function
> > kvm_mem_is_private() doesn't necessarily indicate that the memory
> > isn't shared, but is due to the history and evolution of
> > guest_memfd and the various names it has received. In effect,
> > this function is used to multiplex between the path of a normal
> > page fault and the path of a guest_memfd backed page fault.
> 
> kvm_mem_is_private() is property of the memslot itself. No xarrays
> harmed in the process :)

Ah, I see, but could someone enable CONFIG_GENERIC_PRIVATE_MEM and
related and get confused? Should KVM_GENERIC_MEMORY_ATTRIBUTES=n
depend on !ARM64? Or is it KVM_GMEM_SHARED_MEM that needs to depend on
the generic implementation being off?

Thanks,
Quentin