linux-kernel - Re: [PATCH 00/16] x86/tsc: Try to wrangle PV clocks vs. TSC

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Message-ID: <20250211143919.GBZ6thF2Ryx-D2YpDz@fat_crate.local>
Date: Tue, 11 Feb 2025 15:39:19 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Juergen Gross <jgross@...e.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.amakhalov@...adcom.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	linux-coco@...ts.linux.dev, virtualization@...ts.linux.dev,
	linux-hyperv@...r.kernel.org, jailhouse-dev@...glegroups.com,
	kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
	Nikunj A Dadhania <nikunj@....com>,
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 00/16] x86/tsc: Try to wrangle PV clocks vs. TSC

On Fri, Jan 31, 2025 at 06:17:02PM -0800, Sean Christopherson wrote:
> And if the host provides the core crystal frequency in CPUID.0x15, then KVM
> guests can use that for the APIC timer period instead of manually
> calibrating the frequency.

Hmm, so that part: what's stopping the host from faking the CPUID leaf? I.e.,
I would think that actually doing the work to calibrate the frequency would be
more reliable/harder to fake to a guest than the guest simply reading some
untrusted values from CPUID...

Or are we saying here: oh well, there are so many ways for a normal guest to
be lied to so that we simply do the completely different approach and trust
the HV to be benevolent when we're not dealing with confidential guests which
have all those other things to keep the HV honest?

Just checking the general thinking here.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette