kvm: x86: IA32_ARCH_CAPABILITIES is always supported
authorJim Mattson <jmattson@google.com>
Wed, 9 May 2018 21:29:35 +0000 (14:29 -0700)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 30 May 2018 05:51:50 +0000 (07:51 +0200)
commitb3ce16455c4b4de25e1c0dc23d0d4c03d5324e86
tree839a3eec5de4e2b07ef127c53cdf38da12ca815e
parente765fd97e0c208623b810687a1f8d0a9b280dbbb
kvm: x86: IA32_ARCH_CAPABILITIES is always supported

commit 1eaafe91a0df4157521b6417b3dd8430bf5f52f0 upstream.

If there is a possibility that a VM may migrate to a Skylake host,
then the hypervisor should report IA32_ARCH_CAPABILITIES.RSBA[bit 2]
as being set (future work, of course). This implies that
CPUID.(EAX=7,ECX=0):EDX.ARCH_CAPABILITIES[bit 29] should be
set. Therefore, kvm should report this CPUID bit as being supported
whether or not the host supports it.  Userspace is still free to clear
the bit if it chooses.

For more information on RSBA, see Intel's white paper, "Retpoline: A
Branch Target Injection Mitigation" (Document Number 337131-001),
currently available at https://bugzilla.kernel.org/show_bug.cgi?id=199511.

Since the IA32_ARCH_CAPABILITIES MSR is emulated in kvm, there is no
dependency on hardware support for this feature.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Fixes: 28c1c9fabf48 ("KVM/VMX: Emulate MSR_IA32_ARCH_CAPABILITIES")
Cc: stable@vger.kernel.org
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
arch/x86/kvm/cpuid.c