From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C58C6352026; Wed, 20 May 2026 19:52:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779306772; cv=none; b=eTidalINumHJdfXAztfvLWFB/np1mxZReC+5ANCGdUdZ5Dg7pk8324ZFLmCsizgMYWygA5fTGxs4PODGnvt29J+CNjzhgmKekf5CnYI62MuHzYH+HKxMqzy18iYYhif7XsF0A/8rHdaeInI5oC2cUVHtr7bcho8a5QUyDVQT3b4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779306772; c=relaxed/simple; bh=NfDpWCHNwCELYmlOslr/nQ3b8qR5PBi6+oTunXmBd4o=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=QYC5D8JSPl6n/+5+9NHkYI//2+QsyQeY/7rXut3yAl+2hr3fZaInURYW9mVtVn32UJomgGMR0apdrYTLIOK2GQtJ2/OYYFmaOkrraGcrZCG/3inCOu8VF/jYKMAJnGnq+Aoi2gDKkkMSbga2hksjyqBlfOr0FDs9qdz8V7e1/9o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=DkoJ/w1P; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="DkoJ/w1P" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=NfDpWCHNwCELYmlOslr/nQ3b8qR5PBi6+oTunXmBd4o=; b=DkoJ/w1PPPOjo84eaBmK3YrnUk 9H1j54dtNQaWS9aYKjJOV4LaeDeHzYJ8wV/KR7UGTuxRrRQylp1GWU6k1H+s5w+eWsWwmmqxdybuH gbtoAQOU+y7SeDsiyFcCl2IfVipHbaSO3RiSLrS2oaDiAd+Abl2Smbu93cLi9/vTIw/JMe5RK897z ZBi1VceUgVrtC6pGP+YpmzNDSNqVjVkou6NYUlB7ZUQYrezAJVmCXwGu/ph/hOuIrt1W+H572McTe 9nbmgL7H4CD3jaFTXtPFuXwG8PtQOd7d5siZIRaz2wm2lCJhwyriFq7UeEck7P0P1/rHzSB1WP72H DdxKO+PA==; Received: from 54-240-197-235.amazon.com ([54.240.197.235] helo=freeip.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wPmy6-00000007X3w-0iI8; Wed, 20 May 2026 19:52:38 +0000 Message-ID: Subject: Re: [PATCH v3 05/41] x86/tdx: Override PV calibration routines with CPUID-based calibration From: David Woodhouse To: Sean Christopherson , Kiryl Shutsemau , Paolo Bonzini , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Long Li , Ajay Kaher , Alexey Makhalov , Jan Kiszka , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Juergen Gross , Daniel Lezcano , Thomas Gleixner , John Stultz Cc: Rick Edgecombe , Vitaly Kuznetsov , Broadcom internal kernel review list , Boris Ostrovsky , Stephen Boyd , x86@kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org, linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, Michael Kelley , Tom Lendacky , Nikunj A Dadhania , Thomas Gleixner Date: Wed, 20 May 2026 20:52:36 +0100 In-Reply-To: <20260515191942.1892718-6-seanjc@google.com> References: <20260515191942.1892718-1-seanjc@google.com> <20260515191942.1892718-6-seanjc@google.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-5uq1B3Al8/yI7wiKnpz0" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html --=-5uq1B3Al8/yI7wiKnpz0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2026-05-15 at 12:19 -0700, Sean Christopherson wrote: > When running as a TDX guest, explicitly override the TSC frequency > calibration routine with CPUID-based calibration instead of potentially > relying on a hypervisor-controlled PV routine.=C2=A0 For TDX guests, CPUI= D.0x15 > is always emulated by the TDX-Module, i.e. the information from CPUID is > more trustworthy than the information provided by the hypervisor. >=20 > To maintain backwards compatibility with TDX guest kernels that use nativ= e > calibration, and because it's the least awful option, retain > native_calibrate_tsc()'s stuffing of the local APIC bus period using the > core crystal frequency.=C2=A0 While it's entirely possible for the hyperv= isor > to emulate the APIC timer at a different frequency than the core crystal > frequency, the commonly accepted interpretation of Intel's SDM is that AP= IC > timer runs at the core crystal frequency when that latter is enumerated v= ia > CPUID: >=20 > =C2=A0 The APIC timer frequency will be the processor=E2=80=99s bus clock= or core > =C2=A0 crystal clock frequency (when TSC/core crystal clock ratio is enum= erated > =C2=A0 in CPUID leaf 0x15). >=20 > If the hypervisor is malicious and deliberately runs the APIC timer at th= e > wrong frequency, nothing would stop the hypervisor from modifying the > frequency at any time, i.e. attempting to manually calibrate the frequenc= y > out of paranoia would be futile. >=20 > Deliberately leave the CPU frequency calibration routine as is, since the > TDX-Module doesn't provide any guarantees with respect to CPUID.0x16. >=20 > Opportunistically add a comment explaining that CoCo TSC initialization > needs to come after hypervisor specific initialization. >=20 > Cc: Kirill A. Shutemov > Signed-off-by: Sean Christopherson I don't much like stuffing the lapic_timer_period... but I'll give you 'least awful option'. For now. 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