Re: [PATCH v13 17/20] KVM: arm64: Enable host mapping of shared guest_memfd memory

[Fuad Tabba <tabba@google.com>]

Hi Marc,

On Fri, 11 Jul 2025 at 15:25, Marc Zyngier <maz@kernel.org> wrote:
>
> On Wed, 09 Jul 2025 11:59:43 +0100,
> Fuad Tabba <tabba@google.com> wrote:
> >
> > Enable host userspace mmap support for guest_memfd-backed memory on
> > arm64. This change provides arm64 with the capability to map guest
> > memory at the host directly from guest_memfd:
> >
> > * Define kvm_arch_supports_gmem_mmap() for arm64: The
> >   kvm_arch_supports_gmem_mmap() macro is defined for arm64 to be true if
> >   CONFIG_KVM_GMEM_SUPPORTS_MMAP is enabled. For existing arm64 KVM VM
> >   types that support guest_memfd, this enables them to use guest_memfd
> >   with host userspace mappings. This provides a consistent behavior as
> >   there are currently no arm64 CoCo VMs that rely on guest_memfd solely
> >   for private, non-mappable memory. Future arm64 VM types can override
> >   or restrict this behavior via the kvm_arch_supports_gmem_mmap() hook
> >   if needed.
> >
> > * Select CONFIG_KVM_GMEM_SUPPORTS_MMAP in arm64 Kconfig.
> >
> > * Enforce KVM_MEMSLOT_GMEM_ONLY for guest_memfd on arm64: Compile and
> >   runtime checks are added to ensure that if guest_memfd is enabled on
> >   arm64, KVM_GMEM_SUPPORTS_MMAP must also be enabled. This means
> >   guest_memfd-backed memory slots on arm64 are currently only supported
> >   if they are intended for shared memory use cases (i.e.,
> >   kvm_memslot_is_gmem_only() is true). This design reflects the current
> >   arm64 KVM ecosystem where guest_memfd is primarily being introduced
> >   for VMs that support shared memory.
> >
> > Reviewed-by: James Houghton <jthoughton@google.com>
> > Reviewed-by: Gavin Shan <gshan@redhat.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h | 4 ++++
> >  arch/arm64/kvm/Kconfig            | 1 +
> >  arch/arm64/kvm/mmu.c              | 8 ++++++++
> >  3 files changed, 13 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index d27079968341..bd2af5470c66 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -1675,5 +1675,9 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt);
> >  void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res0, u64 *res1);
> >  void check_feature_map(void);
> >
> > +#ifdef CONFIG_KVM_GMEM
> > +#define kvm_arch_supports_gmem(kvm) true
> > +#define kvm_arch_supports_gmem_mmap(kvm) IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP)
> > +#endif
> >
> >  #endif /* __ARM64_KVM_HOST_H__ */
> > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> > index 713248f240e0..28539479f083 100644
> > --- a/arch/arm64/kvm/Kconfig
> > +++ b/arch/arm64/kvm/Kconfig
> > @@ -37,6 +37,7 @@ menuconfig KVM
> >       select HAVE_KVM_VCPU_RUN_PID_CHANGE
> >       select SCHED_INFO
> >       select GUEST_PERF_EVENTS if PERF_EVENTS
> > +     select KVM_GMEM_SUPPORTS_MMAP
> >       help
> >         Support hosting virtualized guest machines.
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 71f8b53683e7..b92ce4d9b4e0 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -2274,6 +2274,14 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> >       if ((new->base_gfn + new->npages) > (kvm_phys_size(&kvm->arch.mmu) >> PAGE_SHIFT))
> >               return -EFAULT;
> >
> > +     /*
> > +      * Only support guest_memfd backed memslots with mappable memory, since
> > +      * there aren't any CoCo VMs that support only private memory on arm64.
> > +      */
> > +     BUILD_BUG_ON(IS_ENABLED(CONFIG_KVM_GMEM) && !IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP));
> > +     if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
> > +             return -EINVAL;
> > +
> >       hva = new->userspace_addr;
> >       reg_end = hva + (new->npages << PAGE_SHIFT);
> >
>
> Honestly, I don't see the point in making CONFIG_KVM_GMEM a buy in. We
> have *no* configurability for KVM/arm64, the only exception being the
> PMU support, and that has been a pain at every step of the way.
>
> Either KVM is enabled, and it comes with "batteries included", or it's
> not. Either way, we know exactly what we're getting, and it makes
> reproducing problems much easier.

Batteries included is always best I think (all the times I got
disappointed as a kid..... sight :) ). I'll always enable guest_memfd
when KVM is enabled on arm64.

Cheers,
/fuad

> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.