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 DF9604C0420 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=nC4oBn4cJxKkZyffARp111vO9jv76ub7Ra9ST5kfzskoax27PvfKwUDl/A28fVmlU9y5/mRqDGiqKRU/mO54NRFwRWfx600EFvrDIddfPhUl/V5tRM3vUPWbJJmGPrzqGyfYRNi8vfxSA4atjgd1lwYCcnxJ6n5f/f51qke6p0M= 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=Mj1qmozu; 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="Mj1qmozu" Received: by mail-pl1-f201.google.com with SMTP id d9443c01a7336-2bf30576aa3so41043905ad.3 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=lists.linux.dev; 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=Mj1qmozuf2/8NpH7QJl3TyXqMQJdic+lSW7lnrGmS578TsfFSnVP5UqW7ltRkS8RFm FVcOjRjxymT8j4oE71GdVYBvRc85gsdHPmfDj4MVwj/k3gnMKsITI4xp23tfTPnmXs8V ND29mYFmZ1e86VXvpvMjiqwW7DuwfETc7huVRFC+xkNTGDK0uf6Dj8/bDUzMclqgmvq/ DPt1vr9QLdEd+hQuApCdxaTWhoKFmspwpfcntyqV/0N5ILatkh8KN3UC28IyXAPBHGy2 jtpA5X/xCJ8BHO/LoOyWLCNEcFx7H7dYkatEY0H//KfN3enm+uxara9Ihh/mFVSRowJR y9kQ== 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=grKoB4MPP4U1sYiDNj++CcE/1Ke1zsHE76Ma0KBUfRLZpKuC5dgCMoS9jF5x4ay0I6 3adlNsaMSp4EFxBkosrsNuHQ26iQs6EWnARZywqDL3v68mGXbBYxe73b3w2Zx+NKCsWl oNZYOh/8B3O2nzSyNCQFWZwxJl1a8ooTKZQB2rogc0UkFTtRwiRjS9bpJPkuIPpuinr2 Bl9vGB5ZMvqvXvb83DPeTD8YTrSyO3LZt2DRETP5qfr43OwRQ+N12qWHzButVS5tLO78 NQ5a2CYqA2CoIvkdgnG5YG2WlQuM9qPuYiBG6qZKfk6w442CFrB7fRYbRgPKRWcWfmZ9 c8KA== X-Forwarded-Encrypted: i=1; AFNElJ+/zL77Yeez3Pzu1x8BlB6wCPfw3pHinW7YZL3UwVf3kzidEXPhyKi/QekMcS/eZlFya2QhAB6yoBaO@lists.linux.dev X-Gm-Message-State: AOJu0YzJ/eq+5qGQnUU5DqkVuZxjAwY9at1JYJkkW7R6x/wPnl5Lp6I1 6f8zNy3ZCzZnupWR+hRhPzbj1ss8lqlO3grgY++atJgyLorWZAoLRQP7QjimRMp+azn3ALcXxDi 8yHNomg== 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-coco@lists.linux.dev 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.