From: Sean Christopherson <seanjc@google.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.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>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
graf@amazon.de, Ajay Kaher <ajay.kaher@broadcom.com>,
Alexey Makhalov <alexey.makhalov@broadcom.com>,
Colin Percival <cperciva@tarsnap.com>
Subject: Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host
Date: Thu, 21 Aug 2025 09:26:36 -0700 [thread overview]
Message-ID: <aKdIvHOKCQ14JlbM@google.com> (raw)
In-Reply-To: <20250816101308.2594298-1-dwmw2@infradead.org>
On Sat, Aug 16, 2025, David Woodhouse wrote:
> In https://lkml.org/lkml/2008/10/1/246 VMware proposed a generic standard
> for harmonising CPUID between hypervisors. It was mostly shot down in
> flames, but the generic timing leaf at 0x4000_0010 didn't quite die.
>
> Mostly the hypervisor leaves at 0x4000_0xxx are very hypervisor-specific,
> but XNU and FreeBSD as guests will look for 0x4000_0010 unconditionally,
> under any hypervisor. The EC2 Nitro hypervisor has also exposed TSC
> frequency information in this leaf, since 2020.
>
> As things stand, KVM guests have to reverse-calculate the TSC frequency
> from the mul/shift information given to them in the KVM clock to convert
> ticks into nanoseconds, with a corresponding loss of precision.
I would rather have the VMM use the Intel-define CPUID.0x15 to enumerate the
TSC frequency. I would also love, love, love reviews on that series.
https://lore.kernel.org/all/20250227021855.3257188-36-seanjc@google.com
next prev parent reply other threads:[~2025-08-21 16:26 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-08-16 10:09 [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host David Woodhouse
2025-08-16 10:10 ` [PATCH v2 1/3] KVM: x86: Restore caching of KVM CPUID base David Woodhouse
2025-08-16 10:10 ` [PATCH v2 2/3] KVM: x86: Provide TSC frequency in "generic" timing infomation CPUID leaf David Woodhouse
2025-12-16 20:27 ` Sean Christopherson
2025-12-16 21:10 ` Doug Covelli
2025-12-16 22:59 ` Sean Christopherson
2025-08-16 10:10 ` [PATCH v2 3/3] x86/kvm: Obtain TSC frequency from CPUID if present David Woodhouse
2025-08-21 16:26 ` Sean Christopherson [this message]
2025-08-21 17:37 ` [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host David Woodhouse
2025-08-21 19:27 ` Sean Christopherson
2025-08-21 20:42 ` David Woodhouse
2025-08-21 20:48 ` Sean Christopherson
2025-08-21 21:10 ` David Woodhouse
2025-08-22 1:57 ` Colin Percival
2025-08-22 1:57 ` Colin Percival
2025-08-26 19:30 ` Sean Christopherson
2025-08-27 9:30 ` David Woodhouse
2025-08-28 23:40 ` Sean Christopherson
2025-08-29 9:50 ` David Woodhouse
2025-08-29 11:08 ` Durrant, Paul
2025-08-29 11:19 ` David Woodhouse
2025-08-29 20:36 ` Sean Christopherson
2025-09-02 8:31 ` David Woodhouse
2025-09-02 17:49 ` Sean Christopherson
2025-09-02 18:23 ` David Woodhouse
2025-09-04 11:59 ` Sean Christopherson
2025-09-04 12:14 ` David Woodhouse
2025-09-04 13:25 ` Sean Christopherson
2025-09-04 13:51 ` David Woodhouse
2025-09-05 7:57 ` Sean Christopherson
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