Re: [PATCH v4 04/10] KVM: guest_memfd: Add KVM capability to check if guest_memfd is shared

[Peter Xu <peterx@redhat.com>]

On Tue, Feb 18, 2025 at 05:24:54PM +0000, Fuad Tabba wrote:
> Add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which indicates
> that the VM supports shared memory in guest_memfd, or that the
> host can create VMs that support shared memory. Supporting shared
> memory implies that memory can be mapped when shared with the
> host.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  include/uapi/linux/kvm.h | 1 +
>  virt/kvm/kvm_main.c      | 4 ++++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 45e6d8fca9b9..117937a895da 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -929,6 +929,7 @@ struct kvm_enable_cap {
>  #define KVM_CAP_PRE_FAULT_MEMORY 236
>  #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
>  #define KVM_CAP_X86_GUEST_MODE 238
> +#define KVM_CAP_GMEM_SHARED_MEM 239

I think SHARED_MEM is ok.  Said that, to me the use case in this series is
more about "in-place" rather than "shared".

In comparison, what I'm recently looking at is a "more" shared mode of
guest-memfd where it works almost like memfd.  So all pages will be shared
there.

That helps me e.g. for the N:1 kvm binding issue I mentioned in another
email (in one of my relies in previous version), in which case I want to
enable gmemfd folios to be mapped more than once in a process.

That'll work there as long as it's fully shared, because all things can be
registered in the old VA way, then there's no need to have N:1 restriction.
IOW, gmemfd will still rely on mmu notifier for tearing downs, and the
gmem->bindings will always be empty.

So if this one would be called "in-place", then I'll have my use case as
"shared".

I don't want to add any burden to your series, I think I can still make
that one "shared-full"..  So it's more of a pure comment just in case you
also think "in-place" suites more, or any name you think can identify
"in-place conversions" use case and "complete sharable" use cases.

Please also feel free to copy me for newer posts.  I'd be more than happy
to know when gmemfd will have a basic fault() function.

Thanks,

-- 
Peter Xu