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Message-ID: <Zktd8QHU84_EdaNb@google.com>
Date: Mon, 20 May 2024 07:28:01 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: zhoushuling@...wei.com
Cc: pbonzini@...hat.com, weiqi4@...wei.com, wanpengli@...cent.com,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: LAPIC: Fix an inversion error when a negative value
assigned to lapic_timer.timer_advance_ns
On Mon, May 20, 2024, zhoushuling@...wei.com wrote:
> From: Shuling Zhou <zhoushuling@...wei.com>
>
> After 'commit 0e6edceb8f18 ("KVM: LAPIC: Fix lapic_timer_advance_ns
> parameter overflow")',a negative value can be assigned to
> lapic_timer_advance_ns, when it is '-1', the kvm_create_lapic()
> will judge it and turns on adaptive tuning of timer advancement.
> However, when lapic_timer_advance_ns=-2, it will be assigned to
> an uint variable apic->lapic_timer.timer_advance_ns, the
> apic->lapic_timer.timer_advance_ns of each vCPU will become a huge
> value. When a VM is started, the VM is stuck in the
> "
> [ 2.669717] ACPI: Core revision 20130517
> [ 2.672378] ACPI: All ACPI Tables successfully acquired
> [ 2.673309] ftrace: allocating 29651 entries in 116 pages
> [ 2.698797] Enabling x2apic
> [ 2.699431] Enabled x2apic
> [ 2.700160] Switched APIC routing to physical x2apic.
> [ 2.701644] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
> [ 2.702575] smpboot: CPU0: Intel(R) Xeon(R) Platinum 8378A CPU @ 3.00GHz (fam: 06, model: 6a, stepping: 06)
> ..........
> "
>
> 'Fixes: 0e6edceb8f18 ("KVM: LAPIC: Fix lapic_timer_advance_ns
> parameter overflow")'
Fixes: 0e6edceb8f18 ("KVM: LAPIC: Fix lapic_timer_advance_ns parameter overflow")
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Wanpeng Li <wanpengli@...cent.com>
> Signed-off-by: Shuling Zhou<zhoushuling@...wei.com>
There should be whitespace between your name and email.
> ---
> arch/x86/kvm/lapic.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index ebf41023be38..5feeb889ddb6 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2848,7 +2848,7 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
> if (timer_advance_ns == -1) {
> apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_NS_INIT;
> lapic_timer_advance_dynamic = true;
> - } else {
> + } else if (timer_advance_ns >= 0) {
> apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> lapic_timer_advance_dynamic = false;
> }
Wouldn't it be more appropriate to treat any negative value as "dynamic"? The
comment above the module param also needs to be updated.
Oof, and lapic_timer_advance_dynamic is a global, which yields behavior that is
nearly impossible to document. So I think we want this, over two patches?
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index ebf41023be38..3a1bcfbe3e93 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -59,7 +59,6 @@
#define MAX_APIC_VECTOR 256
#define APIC_VECTORS_PER_REG 32
-static bool lapic_timer_advance_dynamic __read_mostly;
#define LAPIC_TIMER_ADVANCE_ADJUST_MIN 100 /* clock cycles */
#define LAPIC_TIMER_ADVANCE_ADJUST_MAX 10000 /* clock cycles */
#define LAPIC_TIMER_ADVANCE_NS_INIT 1000
@@ -1854,7 +1853,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
trace_kvm_wait_lapic_expire(vcpu->vcpu_id, guest_tsc - tsc_deadline);
- if (lapic_timer_advance_dynamic) {
+ if (apic->lapic_timer.timer_advance_dynamic) {
adjust_lapic_timer_advance(vcpu, guest_tsc - tsc_deadline);
/*
* If the timer fired early, reread the TSC to account for the
@@ -2845,12 +2844,12 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
hrtimer_init(&apic->lapic_timer.timer, CLOCK_MONOTONIC,
HRTIMER_MODE_ABS_HARD);
apic->lapic_timer.timer.function = apic_timer_fn;
- if (timer_advance_ns == -1) {
+ if (timer_advance_ns < 0) {
apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_NS_INIT;
- lapic_timer_advance_dynamic = true;
+ apic->lapic_timer.timer_advance_dynamic = true;
} else {
apic->lapic_timer.timer_advance_ns = timer_advance_ns;
- lapic_timer_advance_dynamic = false;
+ apic->lapic_timer.timer_advance_dynamic = false;
}
/*
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 0a0ea4b5dd8c..6fb3b16a2754 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -54,6 +54,7 @@ struct kvm_timer {
u32 timer_advance_ns;
atomic_t pending; /* accumulated triggered timers */
bool hv_timer_in_use;
+ bool timer_advance_dynamic;
};
struct kvm_lapic {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a2b62169e09a..60e86607056e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -165,10 +165,11 @@ static u32 __read_mostly tsc_tolerance_ppm = 250;
module_param(tsc_tolerance_ppm, uint, 0644);
/*
- * lapic timer advance (tscdeadline mode only) in nanoseconds. '-1' enables
- * adaptive tuning starting from default advancement of 1000ns. '0' disables
- * advancement entirely. Any other value is used as-is and disables adaptive
- * tuning, i.e. allows privileged userspace to set an exact advancement time.
+ * lapic timer advance (tscdeadline mode only) in nanoseconds. Any negative
+ * value enable adaptive tuning starting from default advancement of 1000ns.
+ * '0' disables advancement entirely. Any postive value is used as-is and
+ * disables adaptive tuning, i.e. allows privileged userspace to set an exact
+ * advancement time.
*/
static int __read_mostly lapic_timer_advance_ns = -1;
module_param(lapic_timer_advance_ns, int, 0644);
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