[RFC PATCH 1/8] kvm: x86: MSR for setting up scheduler info shared memory

Vineeth Pillai (Google)
2023-12-14T02:47:35Z

Implement a kvm MSR that guest uses to provide the GPA of shared memory
for communicating the scheduling information between host and guest.

wrmsr(0) disables the feature. wrmsr(valid_gpa) enables the feature and
uses the gpa for further communication.

Also add a new cpuid feature flag for the host to advertise the feature
to the guest.

Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
---
arch/x86/include/asm/kvm_host.h | 25 ++++++++++++
arch/x86/include/uapi/asm/kvm_para.h | 24 +++++++++++
arch/x86/kvm/Kconfig | 12 ++++++
arch/x86/kvm/cpuid.c | 2 +
arch/x86/kvm/x86.c | 61 ++++++++++++++++++++++++++++
include/linux/kvm_host.h | 5 +++
6 files changed, 129 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f72b30d2238a..f89ba1f07d88 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -987,6 +987,18 @@ struct kvm_vcpu_arch {
/* Protected Guests */
bool guest_state_protected;

+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+ /*
+ * MSR to setup a shared memory for scheduling
+ * information sharing between host and guest.
+ */
+ struct {
+ enum kvm_vcpu_boost_state boost_status;
+ u64 msr_val;
+ struct gfn_to_hva_cache data;
+ } pv_sched;
+#endif
+
/*
* Set when PDPTS were loaded directly by the userspace without
* reading the guest memory
@@ -2217,4 +2229,17 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
*/
#define KVM_EXIT_HYPERCALL_MBZ GENMASK_ULL(31, 1)

+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+static inline bool kvm_arch_vcpu_pv_sched_enabled(struct kvm_vcpu_arch *arch)
+{
+ return arch->pv_sched.msr_val;
+}
+
+static inline void kvm_arch_vcpu_set_boost_status(struct kvm_vcpu_arch *arch,
+ enum kvm_vcpu_boost_state boost_status)
+{
+ arch->pv_sched.boost_status = boost_status;
+}
+#endif
+
#endif /* _ASM_X86_KVM_HOST_H */
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 6e64b27b2c1e..6b1dea07a563 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -36,6 +36,7 @@
#define KVM_FEATURE_MSI_EXT_DEST_ID 15
#define KVM_FEATURE_HC_MAP_GPA_RANGE 16
#define KVM_FEATURE_MIGRATION_CONTROL 17
+#define KVM_FEATURE_PV_SCHED 18

#define KVM_HINTS_REALTIME 0

@@ -58,6 +59,7 @@
#define MSR_KVM_ASYNC_PF_INT 0x4b564d06
#define MSR_KVM_ASYNC_PF_ACK 0x4b564d07
#define MSR_KVM_MIGRATION_CONTROL 0x4b564d08
+#define MSR_KVM_PV_SCHED 0x4b564da0

struct kvm_steal_time {
__u64 steal;
@@ -150,4 +152,26 @@ struct kvm_vcpu_pv_apf_data {
#define KVM_PV_EOI_ENABLED KVM_PV_EOI_MASK
#define KVM_PV_EOI_DISABLED 0x0

+/*
+ * VCPU boost state shared between the host and guest.
+ */
+enum kvm_vcpu_boost_state {
+ /* Priority boosting feature disabled in host */
+ VCPU_BOOST_DISABLED = 0,
+ /*
+ * vcpu is not explicitly boosted by the host.
+ * (Default priority when the guest started)
+ */
+ VCPU_BOOST_NORMAL,
+ /* vcpu is boosted by the host */
+ VCPU_BOOST_BOOSTED
+};
+
+/*
+ * Structure passed in via MSR_KVM_PV_SCHED
+ */
+struct pv_sched_data {
+ __u64 boost_status;
+};
+
#endif /* _UAPI_ASM_X86_KVM_PARA_H */
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 89ca7f4c1464..dbcba73fb508 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -141,4 +141,16 @@ config KVM_XEN
config KVM_EXTERNAL_WRITE_TRACKING
bool

+config PARAVIRT_SCHED_KVM
+ bool "Enable paravirt scheduling capability for kvm"
+ depends on KVM
+ help
+ Paravirtualized scheduling facilitates the exchange of scheduling
+ related information between the host and guest through shared memory,
+ enhancing the efficiency of vCPU thread scheduling by the hypervisor.
+ An illustrative use case involves dynamically boosting the priority of
+ a vCPU thread when the guest is executing a latency-sensitive workload
+ on that specific vCPU.
+ This config enables paravirt scheduling in the kvm hypervisor.
+
endif # VIRTUALIZATION
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 7bdc66abfc92..960ef6e869f2 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1113,6 +1113,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
(1 << KVM_FEATURE_POLL_CONTROL) |
(1 << KVM_FEATURE_PV_SCHED_YIELD) |
(1 << KVM_FEATURE_ASYNC_PF_INT);
+ if (IS_ENABLED(CONFIG_PARAVIRT_SCHED_KVM))
+ entry->eax |= (1 << KVM_FEATURE_PV_SCHED);

if (sched_info_on())
entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7bcf1a76a6ab..0f475b50ac83 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3879,6 +3879,33 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
return 1;
break;

+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+ case MSR_KVM_PV_SCHED:
+ if (!guest_pv_has(vcpu, KVM_FEATURE_PV_SCHED))
+ return 1;
+
+ if (!(data & KVM_MSR_ENABLED))
+ break;
+
+ if (!(data & ~KVM_MSR_ENABLED)) {
+ /*
+ * Disable the feature
+ */
+ vcpu->arch.pv_sched.msr_val = 0;
+ kvm_set_vcpu_boosted(vcpu, false);
+ } if (!kvm_gfn_to_hva_cache_init(vcpu->kvm,
+ &vcpu->arch.pv_sched.data, data & ~KVM_MSR_ENABLED,
+ sizeof(struct pv_sched_data))) {
+ vcpu->arch.pv_sched.msr_val = data;
+ kvm_set_vcpu_boosted(vcpu, false);
+ } else {
+ pr_warn("MSR_KVM_PV_SCHED: kvm:%p, vcpu:%p, "
+ "msr value: %llx, kvm_gfn_to_hva_cache_init failed!\n",
+ vcpu->kvm, vcpu, data & ~KVM_MSR_ENABLED);
+ }
+ break;
+#endif
+
case MSR_KVM_POLL_CONTROL:
if (!guest_pv_has(vcpu, KVM_FEATURE_POLL_CONTROL))
return 1;
@@ -4239,6 +4266,11 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)

msr_info->data = vcpu->arch.pv_eoi.msr_val;
break;
+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+ case MSR_KVM_PV_SCHED:
+ msr_info->data = vcpu->arch.pv_sched.msr_val;
+ break;
+#endif
case MSR_KVM_POLL_CONTROL:
if (!guest_pv_has(vcpu, KVM_FEATURE_POLL_CONTROL))
return 1;
@@ -9820,6 +9852,29 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
return kvm_skip_emulated_instruction(vcpu);
}

+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+static void record_vcpu_boost_status(struct kvm_vcpu *vcpu)
+{
+ u64 val = vcpu->arch.pv_sched.boost_status;
+
+ if (!kvm_arch_vcpu_pv_sched_enabled(&vcpu->arch))
+ return;
+
+ pagefault_disable();
+ kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.pv_sched.data,
+ &val, offsetof(struct pv_sched_data, boost_status), sizeof(u64));
+ pagefault_enable();
+}
+
+void kvm_set_vcpu_boosted(struct kvm_vcpu *vcpu, bool boosted)
+{
+ kvm_arch_vcpu_set_boost_status(&vcpu->arch,
+ boosted ? VCPU_BOOST_BOOSTED : VCPU_BOOST_NORMAL);
+
+ kvm_make_request(KVM_REQ_VCPU_BOOST_UPDATE, vcpu);
+}
+#endif
+
int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
{
unsigned long nr, a0, a1, a2, a3, ret;
@@ -10593,6 +10648,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
}
if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
record_steal_time(vcpu);
+
+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+ if (kvm_check_request(KVM_REQ_VCPU_BOOST_UPDATE, vcpu))
+ record_vcpu_boost_status(vcpu);
+#endif
+
#ifdef CONFIG_KVM_SMM
if (kvm_check_request(KVM_REQ_SMI, vcpu))
process_smi(vcpu);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9d3ac7720da9..a74aeea55347 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -167,6 +167,7 @@ static inline bool is_error_page(struct page *page)
#define KVM_REQ_VM_DEAD (1 | KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
#define KVM_REQ_UNBLOCK 2
#define KVM_REQ_DIRTY_RING_SOFT_FULL 3
+#define KVM_REQ_VCPU_BOOST_UPDATE 6
#define KVM_REQUEST_ARCH_BASE 8

/*
@@ -2287,4 +2288,8 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
/* Max number of entries allowed for each kvm dirty ring */
#define KVM_DIRTY_RING_MAX_ENTRIES 65536

+#ifdef CONFIG_PARAVIRT_SCHED_KVM
+void kvm_set_vcpu_boosted(struct kvm_vcpu *vcpu, bool boosted);
+#endif
+
#endif
--
2.43.0