linux-kernel - Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove timer drift correction

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: <FABCFBD0-4B76-4662-9F7B-7E1A856BBBB6@infradead.org>
Date: Fri, 19 Apr 2024 20:34:34 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: "Chen, Zide" <zide.chen@...el.com>, Jack Allister <jalliste@...zon.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.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>,
 Shuah Khan <shuah@...nel.org>
CC: Paul Durrant <paul@....org>, kvm@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove timer drift correction

On 19 April 2024 18:13:16 BST, "Chen, Zide" <zide.chen@...el.com> wrote:
>I'm wondering what's the underling theory that we definitely can achieve
>±1ns accuracy? I tested it on a Sapphire Rapids @2100MHz TSC frequency,
>and I can see delta_corrected=2 in ~2% cases.

Hm. Thanks for testing!

So the KVM clock is based on the guest TSC. Given a delta between the guest TSC T and some reference point in time R, the KVM clock is expressed as a(T-R)+r, where little r is the value of the KVM clock when the guest TSC was R, and (a) is the rate of the guest TSC.

When set the clock with KVM_SET_CLOCK_GUEST, we are changing the values of R and r to a new point in time. Call the new ones Q and q respectively.

But we calculate precisely (within 1ns at least) what the KVM clock would have been with the *old* formula, and adjust our new offset (q) so that at our new reference TSC value Q, the formulae give exactly the same result.

And because the *rates* are the same, they should continue to give the same results, ±1ns.

Or such *was* my theory, at least. 

Would be interesting to see it disproven with actual numbers for the old+new pvclock structs, so I can understand where the logic goes wrong.

Were you using frequency scaling?