linux-kernel - Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aKdIvHOKCQ14JlbM@google.com>
Date: Thu, 21 Aug 2025 09:26:36 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, graf@...zon.de, 
	Ajay Kaher <ajay.kaher@...adcom.com>, Alexey Makhalov <alexey.makhalov@...adcom.com>, 
	Colin Percival <cperciva@...snap.com>
Subject: Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest
 and host

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