[PATCH 2/3] KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing CPUID bit when SMT is impossible

[Vitaly Kuznetsov]

Hyper-V 2019 doesn't expose MD_CLEAR CPUID bit to guests when it cannot
guarantee that two virtual processors won't end up running on sibling SMT
threads without knowing about it. This is done as an optimization as in
this case there is nothing the guest can do to protect itself against MDS
and issuing additional flush requests is just pointless. On bare metal the
topology is known, however, when Hyper-V is running nested (e.g. on top of
KVM) it needs an additional piece of information: a confirmation that the
exposed topology (wrt vCPU placement on different SMT threads) is
trustworthy.

NoNonArchitecturalCoreSharing (CPUID 0x40000004 EAX bit 18) is described in
TLFS as follows: "Indicates that a virtual processor will never share a
physical core with another virtual processor, except for virtual processors
that are reported as sibling SMT threads." From KVM we can give such
guarantee in two cases:
- SMT is unsupported or forcefully disabled (just 'disabled' doesn't work
 as it can become re-enabled during the lifetime of the guest).
- vCPUs are properly pinned so the scheduler won't put them on sibling
SMT threads (when they're not reported as such).

This patch reports NoNonArchitecturalCoreSharing bit in to userspace in the
first case. The second case is outside of KVM's domain of responsibility
(as vCPU pinning is actually done by someone who manages KVM's userspace -
e.g. libvirt pinning QEMU threads).

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 arch/x86/include/asm/hyperv-tlfs.h | 7 +++++++
 arch/x86/kvm/hyperv.c              | 4 +++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index af78cd72b8f3..989a1efe7f5e 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h
@@ -170,6 +170,13 @@
 /* Recommend using enlightened VMCS */
 #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED		BIT(14)
 
+/*
+ * Virtual processor will never share a physical core with another virtual
+ * processor, except for virtual processors that are reported as sibling SMT
+ * threads.
+ */
+#define HV_X64_NO_NONARCH_CORESHARING                  BIT(18)
+
 /* Nested features. These are HYPERV_CPUID_NESTED_FEATURES.EAX bits. */
 #define HV_X64_NESTED_GUEST_MAPPING_FLUSH		BIT(18)
 #define HV_X64_NESTED_MSR_BITMAP			BIT(19)
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index fff790a3f4ee..9c187d16a9cd 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -23,6 +23,7 @@
 #include "ioapic.h"
 #include "hyperv.h"
 
+#include <linux/cpu.h>
 #include <linux/kvm_host.h>
 #include <linux/highmem.h>
 #include <linux/sched/cputime.h>
@@ -1864,7 +1865,8 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
 			ent->eax |= HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED;
 			if (evmcs_ver)
 				ent->eax |= HV_X64_ENLIGHTENED_VMCS_RECOMMENDED;
-
+			if (!cpu_smt_possible())
+				ent->eax |= HV_X64_NO_NONARCH_CORESHARING;
 			/*
 			 * Default number of spinlock retry attempts, matches
 			 * HyperV 2016.
-- 
2.20.1