[RFC PATCH 0/2] KVM: x86: Add a new attribute to control dynamic XSTATE components

Chang S. Bae
2022-08-23T23:23:51Z

Hi KVM folks,

While documenting the ARCH_{REQ|GET}_XCOMP_GUEST_PERM options [1], I came
to think of this new attribute as it can provide more consistent way of
enabling dynamic features for QEMU/KVM.

== Background ==

The new x86 arch_prctl() options [2] help to support dynamic AMX state
enabling. As the VCPU permission should be separate, additional options are
added for the userspace VMM like QEMU: ARCH_GET_XCOMP_GUEST_PERM and
ARCH_REQ_XCOMP_GUEST_PERM.

== Problem ==

KVM has already established a set of ioctls to control VM attributes. It
has the attribute to expose the KVM-supported XSTATE components [3].
This interface, however, is not necessarily compatible with those
arch_prctl() options. KVM may choose and expose some of the host-supported
features. Then using the host-provided interface may not comply with this
KVM policy.

== Solution ==

The patchset adds a new attribute to control XSTATE permission:
KVM_X86_XCOMP_GUEST_PERM, and it is available through the
KVM_GET_DEVICE_ATTR/KVM_SET_DEVICE_ATTR APIs. The KVM AMX test is updated
to use this. Then QEMU may switch to keep using the KVM API, which is more
consistent.

[1]: https://lore.kernel.org/lkml/ec95b28f-51a1-a9cf-7d72-a3a865797c7d@intel.com/
[2]: https://docs.kernel.org/x86/xstate.html
[3]: https://gitlab.com/qemu-project/qemu/-/blob/master/linux-headers/asm-x86/kvm.h#L456

Chang S. Bae (2):
KVM: x86: Add a new system attribute for dynamic XSTATE component
selftests: kvm: Use the KVM API to enable dynamic XSTATE features

arch/x86/include/asm/fpu/api.h | 1 +
arch/x86/include/uapi/asm/kvm.h | 1 +
arch/x86/kernel/fpu/xstate.c | 6 ++++
arch/x86/kvm/x86.c | 31 +++++++++++++++++++
tools/arch/x86/include/uapi/asm/kvm.h | 1 +
.../selftests/kvm/lib/x86_64/processor.c | 22 +++++++++----
6 files changed, 56 insertions(+), 6 deletions(-)

base-commit: 1c23f9e627a7b412978b4e852793c5e3c3efc555
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2.17.1