Re: [PATCH RFC v7 02/64] KVM: x86: Add KVM_CAP_UNMAPPED_PRIVATE_MEMORY

[Michael Roth]

On Thu, Dec 22, 2022 at 01:26:25PM +0100, Borislav Petkov wrote:
> On Wed, Dec 14, 2022 at 01:39:54PM -0600, Michael Roth wrote:
> > This mainly indicates to KVM that it should expect all private guest
> > memory to be backed by private memslots. Ideally this would work
> > similarly for others archs, give or take a few additional flags, but
> > for now it's a simple boolean indicator for x86.
> 
> ...
> 
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index c7e9d375a902..cc9424ccf9b2 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1219,6 +1219,7 @@ struct kvm_ppc_resize_hpt {
> >  #define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
> >  #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
> >  #define KVM_CAP_MEMORY_ATTRIBUTES 225
> > +#define KVM_CAP_UNMAPPED_PRIVATE_MEM 240
> 
> Isn't this new cap supposed to be documented somewhere in
> Documentation/virt/kvm/api.rst ?

It should, but this is sort of a placeholder for now. Ideally we'd
re-use the capabilities introduced by UPM patchset rather than introduce
a new one. Originally the UPM patchset had a KVM_CAP_PRIVATE_MEM which
we planned to use to switch between legacy SEV and UPM-based SEV (for
lazy-pinning support) by making it writeable, but that was removed in v10
in favor of KVM_CAP_MEMORY_ATTRIBUTES, which is tied to the new
KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES/KVM_SET_MEMORY_ATTRIBUTES ioctls:

  https://lore.kernel.org/lkml/CA+EHjTxXOdzcP25F57Mtmnb1NWyG5DcyqeDPqzjEOzRUrqH8FQ@xxxxxxxxxxxxxx/

It wasn't clear at the time if that was the right interface to use for
this particular case, so we stuck with the more general
'use-upm/dont-use-upm' semantics originally provided by making
KVM_CAP_UNMAPPED_PRIVATE_MEM/KVM_CAP_PRIVATE_MEM writeable.

But maybe it's okay to just make KVM_CAP_MEMORY_ATTRIBUTES writeable and
require userspace to negotiate it rather than just tying it to
CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES. Or maybe introducing a new
KVM_SET_SUPPORTED_MEMORY_ATTRIBUTES ioctl to pair with
KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES. It sort of makes sense, since userspace
needs to be prepared to deal with KVM_EXIT_MEMORY_FAULTs relating to these
attributes.

-Mike

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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