Re: [PATCH v9 15/17] KVM: Introduce the KVM capability KVM_CAP_GMEM_SHARED_MEM

[Fuad Tabba <tabba@google.com>]

On Wed, 21 May 2025 at 03:47, Gavin Shan <gshan@redhat.com> wrote:
>
> Hi Fuad,
>
> On 5/14/25 2:34 AM, Fuad Tabba wrote:
> > This patch introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
> > indicates that guest_memfd supports shared memory (when enabled by the
> > flag). This support is limited to certain VM types, determined per
> > architecture.
> >
> > This patch also updates the KVM documentation with details on the new
> > capability, flag, and other information about support for shared memory
> > in guest_memfd.
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >   Documentation/virt/kvm/api.rst | 18 ++++++++++++++++++
> >   include/uapi/linux/kvm.h       |  1 +
> >   virt/kvm/kvm_main.c            |  4 ++++
> >   3 files changed, 23 insertions(+)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 47c7c3f92314..86f74ce7f12a 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6390,6 +6390,24 @@ most one mapping per page, i.e. binding multiple memory regions to a single
> >   guest_memfd range is not allowed (any number of memory regions can be bound to
> >   a single guest_memfd file, but the bound ranges must not overlap).
> >
> > +When the capability KVM_CAP_GMEM_SHARED_MEM is supported, the 'flags' field
> > +supports GUEST_MEMFD_FLAG_SUPPORT_SHARED.  Setting this flag on guest_memfd
> > +creation enables mmap() and faulting of guest_memfd memory to host userspace.
> > +
> > +When the KVM MMU performs a PFN lookup to service a guest fault and the backing
> > +guest_memfd has the GUEST_MEMFD_FLAG_SUPPORT_SHARED set, then the fault will
> > +always be consumed from guest_memfd, regardless of whether it is a shared or a
> > +private fault.
> > +
> > +For these memslots, userspace_addr is checked to be the mmap()-ed view of the
> > +same range specified using gmem.pgoff.  Other accesses by KVM, e.g., instruction
> > +emulation, go via slot->userspace_addr.  The slot->userspace_addr field can be
> > +set to 0 to skip this check, which indicates that KVM would not access memory
> > +belonging to the slot via its userspace_addr.
> > +
>
> This paragraph needs to be removed if PATCH[08/17] is going to be dropped.

Done.

Thanks,
/fuad
>
> [PATCH v9 08/17] KVM: guest_memfd: Check that userspace_addr and fd+offset refer to same range
>
> > +The use of GUEST_MEMFD_FLAG_SUPPORT_SHARED will not be allowed for CoCo VMs.
> > +This is validated when the guest_memfd instance is bound to the VM.
> > +
> >   See KVM_SET_USER_MEMORY_REGION2 for additional details.
> >
>
> [...]
>
> Thanks,
> Gavin
>