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 5686D3CBE84; Wed, 20 May 2026 23:45:59 +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=1779320771; cv=none; b=vCqnIrz5hID0Ds3pE6u87scAeIcDeHJhz51lAUEAMrciXENIvSTLGxRcUOchC92+XcS/fOtpLATx9RQYRDkGuSaVttO+RqZoSX0HqW2BlmzSSZpKet99T80Xx/DNIRFOFWPEqb4ia79TMYrMxdtPJjVW/DnmB8EBeOh+T3TetxM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779320771; c=relaxed/simple; bh=LoEMAtKdxWD3Crtub/wIW3SO8OmgI6a6vILD3ZJS1Ks=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=mANpQ2hpw6GfvxSN0l3U0XwYfDXUll+1x+hLQs6Hl+C1f8mdMiSoMO+QQoTe/aheB22a9zBUUz0+FcvP4IGj5/QvjU7kx7KztEEYm6xdhDtCjvRSDM3jlQmr1CeDgPMtahrzgPbr39MnLUmP3S6PgwBdXU/Z7YUx1pevepKC6nI= 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=KGuP+tGa; 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="KGuP+tGa" 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=CDIT4LHc/dZLVw4Pq3pXJSefk+zNidZiNfxkf50m7JQ=; b=KGuP+tGaNAD6aNbiaygC6YoxXR QlzegaOK9ix16FZePm7GRz0VG746bKaw14zTeNO1nwOMuJlkrKFNEKsi294S4KrSGUTUIXtd4aTFk RMTi5+xDgy9U+JRq6TwtDlwEhPve64oUIlR4FXsXOrO7lGM8z4MnU+UJsoZSJrwVDQgeQ+rrEuMaC 50l0uJhzmT/zKAt8U4uXCqbEp2G9h3EM8NtFFi8Q/2m7Uzk2GrOIAdxaMNZeHhto86LTwPPpWITI5 I9754fdkHFA+VhbBMlmZRu2Fdkm7ESX9B9Dq43z1xwtmNyPobNkDZXPtfnv26ThQuusQSnTiIaceU XDfftm5A==; Received: from 54-240-197-227.amazon.com ([54.240.197.227] helo=freeip.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wPqbn-00000007lAX-1Ssh; Wed, 20 May 2026 23:45:51 +0000 Message-ID: <5eb99994b054700d9924f529448730a7dfabe9ce.camel@infradead.org> Subject: Re: [PATCH v3 30/41] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC 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: Thu, 21 May 2026 00:45:48 +0100 In-Reply-To: <20260515191942.1892718-31-seanjc@google.com> References: <20260515191942.1892718-1-seanjc@google.com> <20260515191942.1892718-31-seanjc@google.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-wo/wRt0Fuqirl5QEKL9D" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org 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 --=-wo/wRt0Fuqirl5QEKL9D Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2026-05-15 at 12:19 -0700, Sean Christopherson wrote: > Silently ignore attempts to switch to a paravirt sched_clock when running > as a CoCo guest with trusted TSC.=C2=A0 In hand-wavy theory, a misbehavin= g > hypervisor could attack the guest by manipulating the PV clock to affect > guest scheduling in some weird and/or predictable way.=C2=A0 More importa= ntly, > reading TSC on such platforms is faster than any PV clock, and sched_cloc= k > is all about speed. >=20 > Signed-off-by: Sean Christopherson And kvmclock. And Xen. Are there *any* reasons we'd use a PV sched_clock when the TSC is usable? Reviewed-by: David Woodhouse > --- > =C2=A0arch/x86/kernel/tsc.c | 9 +++++++++ > =C2=A01 file changed, 9 insertions(+) >=20 > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c > index 3c15fc10e501..ac4abfec1f05 100644 > --- a/arch/x86/kernel/tsc.c > +++ b/arch/x86/kernel/tsc.c > @@ -283,6 +283,15 @@ bool using_native_sched_clock(void) > =C2=A0int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stabl= e, > =C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 void (*save)(void), void (*resto= re)(void)) > =C2=A0{ > + /* > + * Don't replace TSC with a PV clock when running as a CoCo guest and > + * the TSC is secure/trusted; PV clocks are emulated by the hypervisor, > + * which isn't in the guest's TCB. > + */ > + if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC) || > + =C2=A0=C2=A0=C2=A0 boot_cpu_has(X86_FEATURE_TDX_GUEST)) > + return -EPERM; > + > =C2=A0 if (!stable) > =C2=A0 clear_sched_clock_stable(); > =C2=A0 --=-wo/wRt0Fuqirl5QEKL9D Content-Type: application/pkcs7-signature; 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