linux-kernel - [PATCH] kvm,x86: Use the refined tsc rate for the guest tsc.

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Message-Id: <20210803075914.3070477-1-suleiman@google.com>
Date:   Tue,  3 Aug 2021 16:59:14 +0900
From:   Suleiman Souhlal <suleiman@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     ssouhlal@...ebsd.org, hikalium@...omium.org,
        senozhatsky@...omium.org, Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Suleiman Souhlal <suleiman@...gle.com>
Subject: [PATCH] kvm,x86: Use the refined tsc rate for the guest tsc.

Prior to this change, the initial tsc rate used by kvm would be
the unrefined rate, instead of the refined rate that is derived
later at boot and used for timekeeping. This can cause time to
advance at different rates between the host and the guest.

Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
---
 arch/x86/kvm/x86.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4116567f3d44..1e59bb326c10 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2199,6 +2199,7 @@ static atomic_t kvm_guest_has_master_clock = ATOMIC_INIT(0);
 #endif
 
 static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
+static DEFINE_PER_CPU(bool, cpu_tsc_khz_changed);
 static unsigned long max_tsc_khz;
 
 static u32 adjust_tsc_khz(u32 khz, s32 ppm)
@@ -2906,6 +2907,14 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 		kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
 		return 1;
 	}
+	/*
+	 * Use the refined tsc_khz instead of the tsc_khz at boot (which was
+	 * not refined yet when we got it), if the tsc frequency hasn't changed.
+	 * If the frequency does change, it does not get refined any further,
+	 * so it is safe to use the one gotten from the notifiers.
+	 */
+	if (!__this_cpu_read(cpu_tsc_khz_changed))
+		tgt_tsc_khz = tsc_khz;
 	if (!use_master_clock) {
 		host_tsc = rdtsc();
 		kernel_ns = get_kvmclock_base_ns();
@@ -8086,6 +8095,8 @@ static void tsc_khz_changed(void *data)
 		khz = freq->new;
 	else if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
 		khz = cpufreq_quick_get(raw_smp_processor_id());
+	if (khz)
+		__this_cpu_write(cpu_tsc_khz_changed, true);
 	if (!khz)
 		khz = tsc_khz;
 	__this_cpu_write(cpu_tsc_khz, khz);
-- 
2.32.0.554.ge1b32706d8-goog