From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f201.google.com (mail-pl1-f201.google.com [209.85.214.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DF9FF4C042E for ; Tue, 9 Jun 2026 17:17:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781025426; cv=none; b=NEu3Ff3JfRdtSDDo13wkAznoA2hm4u9OyUNHnWhi2mcsYH+tnXdoF35DXkfH9GHz3rB06tTe5kKxuYBOfwy3xjt70N5ufWqQCHJFugLxG9GOAFYr51iPSWNyvaFS8OV/ZY+Yxx98h3GdM29b6RwFs4BrDrtcljt/VxJilbJOGc4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781025426; c=relaxed/simple; bh=0yX1le44tc/9XtzmqoIANl5jWHza37di/KrrdJWLzbQ=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=aBXItH8HuXeWWrsovTV6alZ27YUKxo3P236vCBnhZ7vQ/VXoX2iOkU1DLOdyTQHxyUjaoy/roU4/bYK9jyMR7JthmuBDSzc2k+boOeAVs/4wHyM0y/hLnGUgdAyAglGwew/Ha6syimWTctX4iMxr8fxjEpDINCxmSok5yDv6OL4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=s8KBVCtV; arc=none smtp.client-ip=209.85.214.201 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="s8KBVCtV" Received: by mail-pl1-f201.google.com with SMTP id d9443c01a7336-2bf08c2a24bso55542355ad.2 for ; Tue, 09 Jun 2026 10:17:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1781025424; x=1781630224; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=llq4Kn5i8ui4r4izxxTvg7cm7qJE2krNstJXZkbKQn4=; b=s8KBVCtVd1cjGtYZ0XC+KmoyL3dYkMocyXltXuOVX001T1Vhpg3oSCO9LHHYzPEx+R v7llcoRBz8VAxnVxAfrGryebeAP7OpQ2dL9s4xEoeTK7Hil6CKRqTQp50RUmnbKwA//0 sTaGzvjKSq2vSxnkWKrL47I58hGeXqgNHkBOZhk9wtx8Q0K5ydW+9WDyL7dbm9PD/ZVL 5wptLmtUv9nrDXECohxU5pjxw2jVrg4+s0p9MrHtikrGvAKHcvSStexyfHFVVnhlxzER 34Oxm/AN5N2LfYI+z1dMtGtFErPRMECxr8fxI3k4MnXbZmYe176ZX/emaTmKyc1ppxPY AEaA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1781025424; x=1781630224; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=llq4Kn5i8ui4r4izxxTvg7cm7qJE2krNstJXZkbKQn4=; b=EQwsBB5odboxQETo0eypZOECSWCX6fTZGBfLsN9hjV1SwTlzcrdue0ozu0c003eOiG qEPg4FuF0yKnYWaKpgRlvv4j5+0nswSAjXT3n/ne4l9Z2Mbt98aebGBGVEIth2AqxFvG fAQ03Vil1iunZHjUC0/nMOWQVXt0zCFmB2ZS/Nk4J2oCaIuJU22WlACPfTaF2cqWJ7vd yuKRbisENt5LKzZ0xmYFO/uUANkncGMD4UwKsveIe2+AB9IUiJtsgoJgyOFPPBXc2F63 ql72EtCbgmi3xUbFUIwZHUbrdQaNfV7sNCCRfup8kLP7L5M1vcauvmdfhept+pPB/aiF L/NA== X-Forwarded-Encrypted: i=1; AFNElJ8687XmAqcRn1Iiio01UCAbXEHMvHS3omsKntbCzthtraSlXTGaGkoNANNeGZW++1FYfazW0TC9mzFKtMw=@vger.kernel.org X-Gm-Message-State: AOJu0YzNmgDDwHknf4HKqKlrkQnX7ce0WX3Gam9hcYK8v1OE9OLYb116 55r8wq4rUjr7IIH8wm4gzNwelhdnNuwIcwJNMV0hzGzrYiHQJ7YyO0y4cw0QGVOXSX7OAbVcCi+ CvDQcwA== X-Received: from plpg1.prod.google.com ([2002:a17:902:9341:b0:2bd:1ca5:b928]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:196b:b0:2be:3434:4e28 with SMTP id d9443c01a7336-2c2a1c66522mr44737215ad.19.1781025423861; Tue, 09 Jun 2026 10:17:03 -0700 (PDT) Date: Tue, 9 Jun 2026 10:17:03 -0700 In-Reply-To: <87a4t86a0l.ffs@fw13> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260529144435.704127-1-seanjc@google.com> <20260529144435.704127-2-seanjc@google.com> <87fr315fq9.ffs@fw13> <87a4t86a0l.ffs@fw13> Message-ID: Subject: Re: [PATCH v4 01/47] x86/tsc: Never re-calibrate TSC frequency if its exact timing is known From: Sean Christopherson To: Thomas Gleixner Cc: Paolo Bonzini , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Kiryl Shutsemau , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Long Li , Ajay Kaher , Alexey Makhalov , Jan Kiszka , Andy Lutomirski , Peter Zijlstra , Juergen Gross , Daniel Lezcano , John Stultz , "H. Peter Anvin" , Rick Edgecombe , Vitaly Kuznetsov , Broadcom internal kernel review list , Boris Ostrovsky , Stephen Boyd , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev, xen-devel@lists.xenproject.org, David Woodhouse , Tom Lendacky , Nikunj A Dadhania , David Woodhouse , Michael Kelley Content-Type: text/plain; charset="us-ascii" On Fri, Jun 05, 2026, Thomas Gleixner wrote: > On Fri, Jun 05 2026 at 11:04, Sean Christopherson wrote: > But we also should have a check in the TSC init code somewhere which > validates that X86_FEATURE_CONSTANT_TSC is set when > X86_FEATURE_TSC_KNOWN_FREQ is set. X86_FEATURE_TSC_KNOWN_FREQ is useless > w/o X86_FEATURE_CONSTANT_TSC. Ugh, any objection to punting on this for now? KVM and Xen guests will trigger TSC_KNOWN_FREQ without CONSTANT_TSC, thanks to commits: e10f78050323 ("kvmclock: fix TSC calibration for nested guests") 898ec52d2ba0 ("x86/xen/time: Set the X86_FEATURE_TSC_KNOWN_FREQ flag in xen_tsc_khz()") Hyper-V guests might as well? Hyper-V's handling of TSC is weird, even for a hypervisor. Even when the frequency is provided in CPUID by the hypervisor, QEMU at least requires a fairly explicit opt-in to advertise CONSTANT_TSC, presumably to try to prevent users from shooting themselves in the foot.