From: "Chen, Zide" <zide.chen@intel.com>
To: David Woodhouse <dwmw2@infradead.org>, 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,
Dongli Zhang <dongli.zhang@oracle.com>
Subject: Re: [PATCH v2 15/15] KVM: x86: Factor out kvm_use_master_clock()
Date: Wed, 1 May 2024 10:55:46 -0700 [thread overview]
Message-ID: <d788bdab-91b9-4034-a241-ee253ee4fb3e@intel.com> (raw)
In-Reply-To: <20240427111929.9600-16-dwmw2@infradead.org>
On 4/27/2024 4:05 AM, David Woodhouse wrote:
> 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)
typo: 'strut' -> 'struct'
> +{
> + 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) &&
'kvm' should be `vcpu->kvm`
> gtod_is_based_on_tsc(gtod->clock.vclock_mode);
next prev parent reply other threads:[~2024-05-01 17:55 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 ` [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 [this message]
2024-05-01 20:45 ` David Woodhouse
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