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 02/15] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force
Date: Sat, 27 Apr 2024 12:04:59 +0100 [thread overview]
Message-ID: <20240427111929.9600-3-dwmw2@infradead.org> (raw)
In-Reply-To: <20240427111929.9600-1-dwmw2@infradead.org>
From: David Woodhouse <dwmw@amazon.co.uk>
The kvm_guest_time_update() function scales the host TSC frequency to
the guest's using kvm_scale_tsc() and the v->arch.l1_tsc_scaling_ratio
scaling ratio previously calculated for that vCPU. Then calcuates the
scaling factors for the KVM clock itself based on that guest TSC
frequency.
However, it uses kHz as the unit when scaling, and then multiplies by
1000 only at the end.
With a host TSC frequency of 3000MHz and a guest set to 2500MHz, the
result of kvm_scale_tsc() will actually come out at 2,499,999kHz. So
the KVM clock advertised to the guest is based on a frequency of
2,499,999,000 Hz.
By using Hz as the unit from the beginning, the KVM clock would be based
on a more accurate frequency of 2,499,999,999 Hz in this example.
Fixes: 78db6a503796 ("KVM: x86: rewrite handling of scaled TSC for kvmclock")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
---
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/kvm/x86.c | 17 +++++++++--------
arch/x86/kvm/xen.c | 2 +-
3 files changed, 11 insertions(+), 10 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 01c69840647e..8440c4081727 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -887,7 +887,7 @@ struct kvm_vcpu_arch {
gpa_t time;
struct pvclock_vcpu_time_info hv_clock;
- unsigned int hw_tsc_khz;
+ unsigned int hw_tsc_hz;
struct gfn_to_pfn_cache pv_time;
/* set guest stopped flag in pvclock flags field */
bool pvclock_set_guest_stopped_request;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2d2619d3eee4..23281c508c27 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3215,7 +3215,8 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
static int kvm_guest_time_update(struct kvm_vcpu *v)
{
- unsigned long flags, tgt_tsc_khz;
+ unsigned long flags;
+ uint64_t tgt_tsc_hz;
unsigned seq;
struct kvm_vcpu_arch *vcpu = &v->arch;
struct kvm_arch *ka = &v->kvm->arch;
@@ -3252,8 +3253,8 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
/* Keep irq disabled to prevent changes to the clock */
local_irq_save(flags);
- tgt_tsc_khz = get_cpu_tsc_khz();
- if (unlikely(tgt_tsc_khz == 0)) {
+ tgt_tsc_hz = get_cpu_tsc_khz() * 1000LL;
+ if (unlikely(tgt_tsc_hz == 0)) {
local_irq_restore(flags);
kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
return 1;
@@ -3288,14 +3289,14 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
/* With all the info we got, fill in the values */
if (kvm_caps.has_tsc_control)
- tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
- v->arch.l1_tsc_scaling_ratio);
+ tgt_tsc_hz = kvm_scale_tsc(tgt_tsc_hz,
+ v->arch.l1_tsc_scaling_ratio);
- if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
- kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
+ if (unlikely(vcpu->hw_tsc_hz != tgt_tsc_hz)) {
+ kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_hz,
&vcpu->hv_clock.tsc_shift,
&vcpu->hv_clock.tsc_to_system_mul);
- vcpu->hw_tsc_khz = tgt_tsc_khz;
+ vcpu->hw_tsc_hz = tgt_tsc_hz;
kvm_xen_update_tsc_info(v);
}
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 5a83a8154b79..014048c22652 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -2273,7 +2273,7 @@ void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)
entry = kvm_find_cpuid_entry_index(vcpu, function, 2);
if (entry)
- entry->eax = vcpu->arch.hw_tsc_khz;
+ entry->eax = vcpu->arch.hw_tsc_hz / 1000;
}
void kvm_xen_init_vm(struct kvm *kvm)
--
2.44.0
next prev parent reply other threads:[~2024-04-27 11:21 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 ` David Woodhouse [this message]
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 ` [PATCH v2 09/15] KVM: x86: Fix software TSC upscaling in kvm_update_guest_time() David Woodhouse
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20240427111929.9600-3-dwmw2@infradead.org \
--to=dwmw2@infradead.org \
--cc=bp@alien8.de \
--cc=corbet@lwn.net \
--cc=dave.hansen@linux.intel.com \
--cc=dongli.zhang@oracle.com \
--cc=hpa@zytor.com \
--cc=jalliste@amazon.co.uk \
--cc=kvm@vger.kernel.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-kselftest@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=mtosatti@redhat.com \
--cc=oliver.upton@linux.dev \
--cc=paul@xen.org \
--cc=pbonzini@redhat.com \
--cc=seanjc@google.com \
--cc=shuah@kernel.org \
--cc=sveith@amazon.de \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
--cc=zide.chen@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).