linux-kernel - [PATCH V4 1/4] KVM: x86: hyper-v: Calculate APIC bus frequency for Hyper-V

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Message-Id: <d815921fff4c616c8b05bff0fb9377a171c8633b.1711035400.git.reinette.chatre@intel.com>
Date: Thu, 21 Mar 2024 09:37:41 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: isaku.yamahata@...el.com,
	pbonzini@...hat.com,
	erdemaktas@...gle.com,
	vkuznets@...hat.com,
	seanjc@...gle.com,
	vannapurve@...gle.com,
	jmattson@...gle.com,
	mlevitsk@...hat.com,
	xiaoyao.li@...el.com,
	chao.gao@...el.com,
	rick.p.edgecombe@...el.com
Cc: reinette.chatre@...el.com,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH V4 1/4] KVM: x86: hyper-v: Calculate APIC bus frequency for Hyper-V

From: Isaku Yamahata <isaku.yamahata@...el.com>

Remove APIC_BUS_FREQUENCY and calculate it based on nanoseconds per APIC
bus cycle.  APIC_BUS_FREQUENCY is used only for HV_X64_MSR_APIC_FREQUENCY.
The MSR is not frequently read, calculate it every time.

There are two constants related to the APIC bus frequency:
APIC_BUS_FREQUENCY and APIC_BUS_CYCLE_NS.
Only one value is required because one can be calculated from the other:
   APIC_BUS_CYCLES_NS = 1000 * 1000 * 1000 / APIC_BUS_FREQUENCY.

Remove APIC_BUS_FREQUENCY and instead calculate it when needed.
This prepares for support of configurable APIC bus frequency by
requiring to change only a single variable.

Suggested-by: Maxim Levitsky <maximlevitsky@...il.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
Reviewed-by: Maxim Levitsky <maximlevitsky@...il.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>
[reinette: rework changelog]
Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
---

Changes v4:
- Modify subject to match custom: "KVM: x86/hyperv:" ->
  "KVM: x86: hyper-v:" and use standard capitalization for Hyper-V
  ("hyper-v" -> "Hyper-V").
- Rework changelog to remove pronouns ("we").
- Change logic in commit log: "APIC bus cycles per NS" -> "nanoseconds
  per APIC bus cycle".
- Fix typos. (Maxim and Xiaoyao)
- Add Maxim and Xiaoyao's Reviewed-by tags.
- Add Maxim's "Suggested-by".

Changes v3:
- Newly added according to Maxim Levitsky suggestion.

 arch/x86/kvm/hyperv.c | 2 +-
 arch/x86/kvm/lapic.h  | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 8a47f8541eab..1030701db967 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1737,7 +1737,7 @@ static int kvm_hv_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata,
 		data = (u64)vcpu->arch.virtual_tsc_khz * 1000;
 		break;
 	case HV_X64_MSR_APIC_FREQUENCY:
-		data = APIC_BUS_FREQUENCY;
+		data = div64_u64(1000000000ULL, APIC_BUS_CYCLE_NS);
 		break;
 	default:
 		kvm_pr_unimpl_rdmsr(vcpu, msr);
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 0a0ea4b5dd8c..a20cb006b6c8 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -17,7 +17,6 @@
 #define APIC_DEST_MASK			0x800
 
 #define APIC_BUS_CYCLE_NS       1
-#define APIC_BUS_FREQUENCY      (1000000000ULL / APIC_BUS_CYCLE_NS)
 
 #define APIC_BROADCAST			0xFF
 #define X2APIC_BROADCAST		0xFFFFFFFFul
-- 
2.34.1