Hi Peter,
On Fri, 28 Feb 2025 at 08:24, Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > ---
> > 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 understand what you mean. The naming here is to be consistent with
the rest of the series. I don't really have a strong opinion. It means
SHARED_IN_PLACE, but then that would be a mouthful. :)
> 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.
I definitely will. Thanks for your comments.
Cheers,
/fuad
> Thanks,
>
> --
> Peter Xu
>
