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 15/15] KVM: x86: Factor out kvm_use_master_clock()
Date: Sat, 27 Apr 2024 12:05:12 +0100 [thread overview]
Message-ID: <20240427111929.9600-16-dwmw2@infradead.org> (raw)
In-Reply-To: <20240427111929.9600-1-dwmw2@infradead.org>
From: David Woodhouse <dwmw@amazon.co.uk>
Both kvm_track_tsc_matching() and pvclock_update_vm_gtod_copy() make a
decision about whether the KVM clock should be in master clock mode.
They use *different* criteria for the decision though. This isn't really
a problem; it only has the potential to cause unnecessary invocations of
KVM_REQ_MASTERCLOCK_UPDATE if the masterclock was disabled due to TSC
going backwards, or the guest using the old MSR. But it isn't pretty.
Factor the decision out to a single function. And document the historical
reason why it's disabled for guests that use the old MSR_KVM_SYSTEM_TIME.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d6e4469f531a..680b39f17851 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2518,6 +2518,27 @@ static inline bool gtod_is_based_on_tsc(int mode)
}
#endif
+static bool kvm_use_master_clock(strut kvm *kvm)
+{
+ struct kvm_arch *ka = &kvm->arch;
+
+ /*
+ * The 'old kvmclock' check is a workaround (from 2015) for a
+ * SUSE 2.6.16 kernel that didn't boot if the system_time in
+ * its kvmclock was too far behind the current time. So the
+ * mode of just setting the reference point and allowing time
+ * to proceed linearly from there makes it fail to boot.
+ * Despite that being kind of the *point* of the way the clock
+ * is exposed to the guest. By coincidence, the offending
+ * kernels used the old MSR_KVM_SYSTEM_TIME, which was moved
+ * only because it resided in the wrong number range. So the
+ * workaround is activated for *all* guests using the old MSR.
+ */
+ return ka->all_vcpus_matched_tsc &&
+ !ka->backwards_tsc_observed &&
+ !ka->boot_vcpu_runs_old_kvmclock;
+}
+
static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
@@ -2550,7 +2571,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
* To use the masterclock, the host clocksource must be based on TSC
* and all vCPUs must have matching TSC frequencies.
*/
- bool use_master_clock = ka->all_vcpus_matched_tsc &&
+ bool use_master_clock = kvm_use_master_clock(kvm) &&
gtod_is_based_on_tsc(gtod->clock.vclock_mode);
/*
@@ -3089,9 +3110,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
&ka->master_cycle_now);
ka->use_master_clock = host_tsc_clocksource
- && ka->all_vcpus_matched_tsc
- && !ka->backwards_tsc_observed
- && !ka->boot_vcpu_runs_old_kvmclock;
+ && kvm_use_master_clock(kvm);
/*
* When TSC scaling is in use (which can thankfully only happen
--
2.44.0
next prev parent reply other threads:[~2024-04-27 11:21 UTC|newest]
Thread overview: 20+ 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 ` [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
2024-04-27 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 ` David Woodhouse [this message]
2024-05-01 17:55 ` [PATCH v2 15/15] KVM: x86: Factor out kvm_use_master_clock() 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=20240427111929.9600-16-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).