From: David Woodhouse <dwmw2@infradead.org>
To: kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Jonathan Corbet <corbet@lwn.net>,
Sean Christopherson <seanjc@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Paul Durrant <paul@xen.org>, Shuah Khan <shuah@kernel.org>,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-kselftest@vger.kernel.org,
Oliver Upton <oliver.upton@linux.dev>,
Marcelo Tosatti <mtosatti@redhat.com>,
jalliste@amazon.co.uk, sveith@amazon.de, zide.chen@intel.com,
Dongli Zhang <dongli.zhang@oracle.com>
Subject: [PATCH v2 09/15] KVM: x86: Fix software TSC upscaling in kvm_update_guest_time()
Date: Sat, 27 Apr 2024 12:05:06 +0100 [thread overview]
Message-ID: <20240427111929.9600-10-dwmw2@infradead.org> (raw)
In-Reply-To: <20240427111929.9600-1-dwmw2@infradead.org>
From: David Woodhouse <dwmw@amazon.co.uk>
There was some confusion in kvm_update_guest_time() when software needs
to advance the guest TSC.
In master clock mode, there are two points of time which need to be taken
into account. First there is the master clock reference point, stored in
kvm->arch.master_kernel_ns (and associated host TSC ->master_cycle_now).
Secondly, there is the time *now*, at the point kvm_update_guest_time()
is being called.
With software TSC upscaling, the guest TSC is getting further and further
ahead of the host TSC as time elapses. So at time "now", the guest TSC
should be further ahead of the host, than it was at master_kernel_ns.
The adjustment in kvm_update_guest_time() was not taking that into
account, and was only advancing the guest TSC by the appropriate amount
for master_kernel_ns, *not* the current time.
Fix it to calculate them both correctly.
Since the KVM clock reference point in master_kernel_ns might actually
be *earlier* than the reference point used for the guest TSC
(vcpu->last_tsc_nsec), this might lead to a negative delta. Fix the
compute_guest_tsc() function to cope with negative numbers, which
then means there is no need to force a master clock update when the
guest TSC is written.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/kvm/x86.c | 73 +++++++++++++++++++++++++++++++++++-----------
1 file changed, 56 insertions(+), 17 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 89918ba266cd..e09dc44978ea 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2491,10 +2491,19 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz)
static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
{
- u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec,
- vcpu->arch.virtual_tsc_mult,
- vcpu->arch.virtual_tsc_shift);
- tsc += vcpu->arch.this_tsc_write;
+ s64 delta = kernel_ns - vcpu->arch.this_tsc_nsec;
+ u64 tsc = vcpu->arch.this_tsc_write;
+
+ /* pvclock_scale_delta cannot cope with negative deltas */
+ if (delta >= 0)
+ tsc += pvclock_scale_delta(delta,
+ vcpu->arch.virtual_tsc_mult,
+ vcpu->arch.virtual_tsc_shift);
+ else
+ tsc -= pvclock_scale_delta(-delta,
+ vcpu->arch.virtual_tsc_mult,
+ vcpu->arch.virtual_tsc_shift);
+
return tsc;
}
@@ -2505,7 +2514,7 @@ static inline bool gtod_is_based_on_tsc(int mode)
}
#endif
-static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
+static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
struct kvm_arch *ka = &vcpu->kvm->arch;
@@ -2522,12 +2531,9 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
/*
* Request a masterclock update if the masterclock needs to be toggled
- * on/off, or when starting a new generation and the masterclock is
- * enabled (compute_guest_tsc() requires the masterclock snapshot to be
- * taken _after_ the new generation is created).
+ * on/off.
*/
- if ((ka->use_master_clock && new_generation) ||
- (ka->use_master_clock != use_master_clock))
+ if ((ka->use_master_clock != use_master_clock))
kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
@@ -2705,7 +2711,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
- kvm_track_tsc_matching(vcpu, !matched);
+ kvm_track_tsc_matching(vcpu);
}
static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
@@ -3300,8 +3306,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
kernel_ns = get_kvmclock_base_ns();
}
- tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
-
/*
* We may have to catch up the TSC to match elapsed wall clock
* time for two reasons, even if kvmclock is used.
@@ -3313,11 +3317,46 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
* very slowly.
*/
if (vcpu->tsc_catchup) {
- u64 tsc = compute_guest_tsc(v, kernel_ns);
- if (tsc > tsc_timestamp) {
- adjust_tsc_offset_guest(v, tsc - tsc_timestamp);
- tsc_timestamp = tsc;
+ uint64_t now_host_tsc, now_guest_tsc;
+ int64_t adjustment;
+
+ /*
+ * First, calculate what the guest TSC should be at the
+ * time (kernel_ns) which will be placed in the hvclock.
+ * This may be the *current* time, or it may be the time
+ * of the master clock reference. This is 'tsc_timestamp'.
+ */
+ tsc_timestamp = compute_guest_tsc(v, kernel_ns);
+
+ now_guest_tsc = tsc_timestamp;
+ now_host_tsc = host_tsc;
+
+#ifdef CONFIG_X86_64
+ /*
+ * If the master clock was used, calculate what the guest
+ * TSC should be *now* in order to advance to that.
+ */
+ if (use_master_clock) {
+ int64_t now_kernel_ns;
+
+ if (!kvm_get_time_and_clockread(&now_kernel_ns,
+ &now_host_tsc)) {
+ now_kernel_ns = get_kvmclock_base_ns();
+ now_host_tsc = rdtsc();
+ }
+ now_guest_tsc = compute_guest_tsc(v, now_kernel_ns);
}
+#endif
+ /*
+ * Calculate the delta between what the guest TSC *should* be,
+ * and what it actually is according to kvm_read_l1_tsc().
+ */
+ adjustment = now_guest_tsc - kvm_read_l1_tsc(v, now_host_tsc);
+
+ if (adjustment > 0)
+ adjust_tsc_offset_guest(v, adjustment);
+ } else {
+ tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
}
local_irq_restore(flags);
--
2.44.0
next prev parent reply other threads:[~2024-04-27 11:19 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-27 11:04 [RFC PATCH v2] Cleaning up the KVM clock mess David Woodhouse
2024-04-27 11:04 ` [PATCH v2 01/15] KVM: x86/xen: Do not corrupt KVM clock in kvm_xen_shared_info_init() David Woodhouse
2024-05-04 7:42 ` Marcelo Tosatti
2024-05-07 19:08 ` David Woodhouse
2024-04-27 11:04 ` [PATCH v2 02/15] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force David Woodhouse
2024-04-27 11:05 ` [PATCH v2 03/15] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration David Woodhouse
2024-04-27 11:05 ` [PATCH v2 04/15] KVM: selftests: Add KVM/PV clock selftest to prove timer correction David Woodhouse
2024-04-27 11:05 ` [PATCH v2 05/15] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC David Woodhouse
2024-04-27 11:05 ` [PATCH v2 06/15] KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the documentation on TSC migration David Woodhouse
2024-04-27 11:05 ` [PATCH v2 07/15] KVM: x86: Avoid NTP frequency skew for KVM clock on 32-bit host David Woodhouse
2024-04-27 11:05 ` [PATCH v2 08/15] KVM: x86: Fix KVM clock precision in __get_kvmclock() David Woodhouse
2024-04-27 11:05 ` David Woodhouse [this message]
2024-04-27 11:05 ` [PATCH v2 10/15] KVM: x86: Simplify and comment kvm_get_time_scale() David Woodhouse
2024-04-27 11:05 ` [PATCH v2 11/15] KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset() David Woodhouse
2024-04-27 11:05 ` [PATCH v2 12/15] KVM: x86: Improve synchronization in kvm_synchronize_tsc() David Woodhouse
2024-04-27 11:05 ` [PATCH v2 13/15] KVM: x86: Kill cur_tsc_{nsec,offset,write} fields David Woodhouse
2024-05-10 9:03 ` Chenyi Qiang
2024-05-14 13:17 ` David Woodhouse
2024-04-27 11:05 ` [PATCH v2 14/15] KVM: x86: Allow KVM master clock mode when TSCs are offset from each other David Woodhouse
2024-04-27 11:05 ` [PATCH v2 15/15] KVM: x86: Factor out kvm_use_master_clock() David Woodhouse
2024-05-01 17:55 ` Chen, Zide
2024-05-01 20:45 ` David Woodhouse
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