linux-kernel - [PATCH] KVM: Avoid warning "user requested TSC rate below hardware speed" when create VM.

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Date:	Tue, 16 Jun 2015 15:34:27 +0800
From:	Lan Tianyu <tianyu.lan@...el.com>
To:	gleb@...nel.org, pbonzini@...hat.com
Cc:	Lan Tianyu <tianyu.lan@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: Avoid warning "user requested TSC rate below hardware speed" when create VM.

KVM populates max_tsc_khz with tsc_khz at arch init stage on the
constant tsc machine and creates VM with max_tsc_khz as tsc. However,
tsc_khz maybe changed during tsc clocksource driver refines calibration.
This will cause to create VM with slow tsc and produce the following
warning. To fix the issue, compare max_tsc_khz with tsc_khz and
update max_tsc_khz with new value of tsc_khz if it has been changed
when create a VM.

[   94.916906] ------------[ cut here ]------------
[   94.922127] WARNING: CPU: 0 PID: 824 at arch/x86/kvm/vmx.c:2272 vmx_set_tsc_khz+0x3e/0x50()
[   94.931503] user requested TSC rate below hardware speed
[   94.937470] Modules linked in:
[   94.940923] CPU: 0 PID: 824 Comm: qemu-system-x86 Tainted: G      D W       4.1.0-rc3+ #4
[   94.960350]  ffffffff81f453f8 ffff88027e9f3bc8 ffffffff81b5eb8a 0000000000000000
[   94.968721]  ffff88027e9f3c18 ffff88027e9f3c08 ffffffff810e6f8a ffff880200000000
[   94.977103]  00000000001d3300 ffff88027e980000 0000000000000001 ffff88027e980000
[   94.985476] Call Trace:
[   94.988242]  [<ffffffff81b5eb8a>] dump_stack+0x45/0x57
[   94.994020]  [<ffffffff810e6f8a>] warn_slowpath_common+0x8a/0xc0
[   95.000772]  [<ffffffff810e7006>] warn_slowpath_fmt+0x46/0x50
[   95.007222]  [<ffffffff8104676e>] vmx_set_tsc_khz+0x3e/0x50
[   95.013488]  [<ffffffff810112f7>] kvm_set_tsc_khz.part.106+0xa7/0xe0
[   95.020618]  [<ffffffff8101e628>] kvm_arch_vcpu_init+0x208/0x230
[   95.027373]  [<ffffffff81003bf9>] kvm_vcpu_init+0xc9/0x110
[   95.033540]  [<ffffffff81049fd0>] vmx_create_vcpu+0x70/0xc30
[   95.039911]  [<ffffffff81049f80>] ? vmx_create_vcpu+0x20/0xc30
[   95.046476]  [<ffffffff8101dc9e>] kvm_arch_vcpu_create+0x3e/0x60
[   95.053233]  [<ffffffff81009f00>] kvm_vm_ioctl+0x1a0/0x770
[   95.059400]  [<ffffffff8129f395>] ? __fget+0x5/0x200
[   95.064991]  [<ffffffff8115b85f>] ? rcu_irq_exit+0x7f/0xd0
[   95.071157]  [<ffffffff81293448>] do_vfs_ioctl+0x308/0x540
[   95.077323]  [<ffffffff8129f301>] ? expand_files+0x1f1/0x280
[   95.083684]  [<ffffffff8147836b>] ? selinux_file_ioctl+0x5b/0x100
[   95.090538]  [<ffffffff81293701>] SyS_ioctl+0x81/0xa0
[   95.096218]  [<ffffffff81b6a72e>] system_call_fastpath+0x12/0x76
[   95.102974] ---[ end trace 08ade884081d9dd7 ]---

Link: https://bugzilla.kernel.org/show_bug.cgi?id=99861
Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>
---
 arch/x86/kvm/x86.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 43f0df7..6c7fefe 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7814,6 +7814,7 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 {
 	struct page *page;
 	struct kvm *kvm;
+	int cpu;
 	int r;
 
 	BUG_ON(vcpu->kvm == NULL);
@@ -7833,6 +7834,21 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 	}
 	vcpu->arch.pio_data = page_address(page);
 
+	/*
+	 * max_tsc_khz records tsc_khz at arch init stage on the constant tsc
+	 * machine. However, tsc_khz maybe changed during tsc clocksource driver
+	 * refines calibration. This will cause to create VM with slow tsc
+	 * and produce warning. To avoid such case, check whether tsc_khz
+	 * has been changed here and update max_tsc_khz with new value of
+	 * tsc_khz if changed.
+	 */
+	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && max_tsc_khz != tsc_khz) {
+		max_tsc_khz = tsc_khz;
+		pr_debug("kvm: max_tsc_khz is changed to %ld\n", max_tsc_khz);
+		for_each_online_cpu(cpu)
+			smp_call_function_single(cpu, tsc_khz_changed, NULL, 1);
+	}
+
 	kvm_set_tsc_khz(vcpu, max_tsc_khz);
 
 	r = kvm_mmu_create(vcpu);
-- 
1.8.4.rc0.1.g8f6a3e5.dirty

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