[RFC PATCH 2/2] kvm,x86: Report preempt_count to host.

[Suleiman Souhlal <suleiman@google.com>]

When KVM_PREEMPT_COUNT_REPORTING is enabled, the host can use
preempt_count to determine if the guest is in a critical section,
if it also has CONFIG_KVM_HETEROGENEOUS_RT enabled, in order to
use heterogeneous RT VCPU configurations.

Signed-off-by: Suleiman Souhlal <suleiman@google.com>
---
 arch/x86/Kconfig      | 11 +++++++++++
 arch/x86/kernel/kvm.c | 10 ++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 49270655e827..d8b62789df57 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -846,6 +846,17 @@ config PARAVIRT_TIME_ACCOUNTING
 config PARAVIRT_CLOCK
 	bool
 
+config KVM_PREEMPT_COUNT_REPORTING
+	bool "KVM preempt_count reporting to the host"
+	depends on KVM_GUEST && PREEMPT_COUNT
+	default n
+	help
+	  Select this option to enable KVM preempt_count reporting to the host,
+	  which can be useful in cases where some VCPUs are RT and the rest
+	  aren't.
+
+	  If in doubt, say N here.
+
 config JAILHOUSE_GUEST
 	bool "Jailhouse non-root cell support"
 	depends on X86_64 && PCI
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index a26643dc6bd6..7ec53ea3f979 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -363,6 +363,16 @@ static void kvm_guest_cpu_init(void)
 
 	if (has_steal_clock)
 		kvm_register_steal_time();
+
+#ifdef CONFIG_KVM_PREEMPT_COUNT_REPORTING
+	if (kvm_para_has_feature(KVM_FEATURE_PREEMPT_COUNT)) {
+		unsigned long pa;
+
+		pa = slow_virt_to_phys(this_cpu_ptr(&__preempt_count)) |
+		    KVM_MSR_ENABLED;
+		wrmsrl(MSR_KVM_PREEMPT_COUNT, pa);
+	}
+#endif
 }
 
 static void kvm_pv_disable_apf(void)
-- 
2.32.0.432.gabb21c7263-goog