Re: [PATCH 0/3] KVM: x86: export supported_xcr0 via UAPI

[Yang Zhong <yang.zhong@intel.com>]

On Wed, Jan 26, 2022 at 10:22:07AM -0500, Paolo Bonzini wrote:
> While working on the QEMU support for AMX, I noticed that there is no
> equivalent of ARCH_GET_XCOMP_SUPP in the KVM API.  This is important
> because KVM_GET_SUPPORTED_CPUID is meant to be passed (by simple-minded
> VMMs) to KVM_SET_CPUID2, and therefore it cannot include any dynamic
> xsave states that have not been enabled.  Probing the availability of
> dynamic xsave states therefore, requires a new ioctl or arch_prctl.
> 
> In order to avoid moving supported_xcr0 to the kernel from the KVM
> module just for this use, and to ensure that the value can only be
> probed if/after the KVM module has been loaded, this series goes
> for the former option.
> 
> KVM_CHECK_EXTENSION cannot be used because it only has 32 bits of
> output; in order to limit the growth of capabilities and ioctls, the
> series adds a /dev/kvm variant of KVM_{GET,HAS}_DEVICE_ATTR that
> can be used in the future and by other architectures.  It then
> implements it in x86 with just one group (0) and attribute
> (KVM_X86_XCOMP_GUEST_SUPP).
> 
> The corresponding changes to the tests, in patches 1 and 3, are
> designed so that the code will be covered (to the possible extent)
> even when running the tests on systems that do not support AMX.
> However, the patches have not been tested with AMX.
>

  Paolo, thanks for this patchset. I applied this patchset into latest
  Linux release, and verified it from kvm selftest tool and Qemu side
  (In order to verify this easily, I reused the older request permission
   function like kvm selftest did), all work well. thanks!

  Yang
 
> Thanks,
> 
> Paolo
> 
> 
> Paolo Bonzini (3):
>   selftests: kvm: move vm_xsave_req_perm call to amx_test
>   KVM: x86: add system attribute to retrieve full set of supported xsave
>     states
>   selftests: kvm: check dynamic bits against KVM_X86_XCOMP_GUEST_SUPP
> 
>  Documentation/virt/kvm/api.rst                |  4 +-
>  arch/x86/include/uapi/asm/kvm.h               |  3 ++
>  arch/x86/kvm/x86.c                            | 45 +++++++++++++++++++
>  include/uapi/linux/kvm.h                      |  1 +
>  tools/arch/x86/include/uapi/asm/kvm.h         |  3 ++
>  tools/include/uapi/linux/kvm.h                |  1 +
>  .../selftests/kvm/include/kvm_util_base.h     |  1 -
>  .../selftests/kvm/include/x86_64/processor.h  |  1 +
>  tools/testing/selftests/kvm/lib/kvm_util.c    |  7 ---
>  .../selftests/kvm/lib/x86_64/processor.c      | 27 ++++++++---
>  tools/testing/selftests/kvm/x86_64/amx_test.c |  2 +
>  11 files changed, 80 insertions(+), 15 deletions(-)
> 
> -- 
> 2.31.1