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 6A3B126E70E; Sat, 6 Jun 2026 11:22:29 +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=1780744951; cv=none; b=M9T61PZyhL2Dt6IDNSLldplt1QRy1xgT21ZjNzXcIN9d4+0ISotEYjmBBfBSD3bbNlTPTJ35SvtGqE58DpqvP/EGUHEAxFPPl2EMrFVVE19i6RGcWn7OfFyOUSHEig7PqZzMZ9JnHIojDP1odz3dWlH1f+DTcTnYFJUsijlhm3Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780744951; c=relaxed/simple; bh=OfVGc2a/pKhx2S9Bo13sZeYMs22evlGSYnm4dKXkcqc=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=byeddeBfUN/FfOhvPM3W9MWE8J256n+JdOor4nsbkz9W5RV5Sy2Bf7b4z/hnZZIBK/xgFRWkPBCWhDtiGfGKQ+eE2uRuFW0UBv6otEaqwC7VOKb2nS1Y9kjPnRT9quiGWJY1LYfuJaqVKRq3a9EGTgk07xMk3v6hA69GzGEPrno= 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=A0fOi7a1; 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="A0fOi7a1" 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=OfVGc2a/pKhx2S9Bo13sZeYMs22evlGSYnm4dKXkcqc=; b=A0fOi7a1wPHqhPJgXQPl+d6X2b MrJUVQ2mX/sLZ7q5BI/l6evmPyyEnEYrhZP9RPhTVgFJ1aMqHszMWQQdRB28Q2mc2P6mQSb9cNsCx QPXP6EOhrgEMa7Pt29FBHJHzWyKBANwFYZmOcPAOcemYtcIkPqqW0Nz+83u9ZQNO72gw4GWj4SH/e Z0a8CQoDO/ponj6ZYFw4iUQSroWRZ2mQDrJ6fXoCLvnDMO+eBF8uZsAC0Yfd8CR3SsXlbfNTvWnya M3aD+w6MYAwVlGyJNKJKdqCFz/N0GbDAOPB4bXvHM+3jFvCyDQUNcU4JEzOxyDZvK9qtqDXm8DMcs h7RMlFBA==; Received: from [2001:8b0:10b:5:e135:8c80:b1ce:3f2f] (helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wVp6c-0000000AG3E-297u; Sat, 06 Jun 2026 11:22:23 +0000 Message-ID: <8d921e891cb71bf4c9c375df34707488fb06120d.camel@infradead.org> Subject: Re: [PATCH 2/8] KVM: x86/xen: Introduce kvm_xen_has_64bit_shinfo() macro From: David Woodhouse To: David Laight Cc: Sean Christopherson , Paul Durrant , Hyunwoo Kim , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Date: Sat, 06 Jun 2026 12:22:22 +0100 In-Reply-To: <20260606121152.5ab6bf09@pumpkin> References: <20260605143034.3603-1-dwmw2@infradead.org> <20260605143034.3603-3-dwmw2@infradead.org> <20260606103043.0d9a4baf@pumpkin> <63c77cb3e7fb8b9b1ee0baa006760d494f16ce54.camel@infradead.org> <20260606121152.5ab6bf09@pumpkin> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-A99StRgy63IUaiRG2jCt" User-Agent: Evolution 3.60.3 Precedence: bulk X-Mailing-List: linux-kernel@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 --=-A99StRgy63IUaiRG2jCt Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Sat, 2026-06-06 at 12:11 +0100, David Laight wrote: > On Sat, 06 Jun 2026 10:35:45 +0100 > David Woodhouse wrote: >=20 > > On Sat, 2026-06-06 at 10:30 +0100, David Laight wrote: > > > On Fri,=C2=A0 5 Jun 2026 15:17:27 +0100 > > > David Woodhouse wrote: > > > =C2=A0=20 > > > > From: David Woodhouse > > > >=20 > > > > Add a kvm_xen_has_64bit_shinfo() helper macro to replace the repeat= ed > > > > pattern of 'IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode' > > > > throughout the Xen emulation code. The macro uses READ_ONCE() to > > > > ensure a consistent snapshot of the flag, which can be changed by > > > > another vCPU at any time.=C2=A0=20 > > >=20 > > > If another vCPU changes the flag then isn't it all broken? > > > The code seems to need the value to be stable.=C2=A0=20 > >=20 > > Do you believe the explanation in patch 4 to be insufficient? >=20 > (I read patch 2 first...) > Changing the same lines twice is s bit excessive. You mean a "no functional change" cleanup followed by the actual change? No, that's normal. >=20 > There is this bit in #4: >=20 > =C2=A0Even with this fix, the same corruption can occur if 64-bit mode is > =C2=A0latched and the guest switches to 32-bit mode immediately afterward= . >=20 > which, I think, means it is all hopeless really. I'm glad you now think that. I had explained it twice, both in the cover letter and in the commit message of patch 4 that I pointed you at. If a guest switches an *active* shared info page between 32 and 64 bit then we make almost no attempt to convert the contents. All we're doing here is making things prettier by at least being consistent about whether we're treating it as 32-bit or 64-bit for the duration of each hypercall or event. Which is why it says, "This fix is for internal consistency rather than correcting any observable bug." 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