From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from wp530.webpack.hosteurope.de (wp530.webpack.hosteurope.de [80.237.130.52]) (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 B2F0F1D417B; Mon, 7 Oct 2024 14:04:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=80.237.130.52 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728309870; cv=none; b=F1j9gYIa/o5tiX4Sj7OUbYbgQVjWr/lmeYvrRWakwLCzNSQcvS5IBC0MUBLjDQzJXzLYOAgIY1uGgb1IOT7YQ4Z+9w/egfSRTR8Lt36AOQC3zGhwbIkzHbyNuli/rQoBXsCDLcdPKxqBwNPbYH616t01psr7H1ozibVHivZ1Qs0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728309870; c=relaxed/simple; bh=rYFip1NklXra7HzTO1VszgtuzTsU+opowy+ALEYJL4c=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=lqEncr0jfU/B6Z0n6XfrsejBvcWduP+RxOSjf+9oq6lGerRDtiD5M1/XZg1TIYl6o0gRhdMQENaYWWX/nKeKGlEWQ9pxOgspyL1t1EY58aJo/6XimlyuaYHjcMrTUl05LSn9CV4Dh3rqkOR2sRiJ7Gp3zZ1dqhn6INZgFfQx/9o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=leemhuis.info; spf=pass smtp.mailfrom=leemhuis.info; dkim=pass (2048-bit key) header.d=leemhuis.info header.i=@leemhuis.info header.b=QexIW1ta; arc=none smtp.client-ip=80.237.130.52 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=leemhuis.info Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=leemhuis.info Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=leemhuis.info header.i=@leemhuis.info header.b="QexIW1ta" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=leemhuis.info; s=he214686; h=Content-Transfer-Encoding:Content-Type: In-Reply-To:Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:From:Sender:Reply-To:Subject:Date:Message-ID:To:Cc:MIME-Version: Content-Type:Content-Transfer-Encoding:Content-ID:Content-Description: In-Reply-To:References; bh=5BN/qPr4UHro54JgcseHqfbV3VVI2TKoS2hEvYZX07s=; t=1728309868; x=1728741868; b=QexIW1taqCqeM9s91UlHbuoMAukJ6TLBLJ/iQXrO6BykbXs YRq2BCigAw/rGcJqfqdxHmUsH/MCg+TgQXtWiAhgaBthPzTx5BPlmyDCz9BZHMS/vxP9z8hdC+N7B 9b/rRABkQrmaXtxmHKGrsvLuLm8WDDH6kw387B9N+YUs2CIjwCsKmMQy76VIerngmj7oZV2lwb/mm lNwWrZfaECe9P+8LnSNcAB5tLlMWzYG/vrUF6aDWOtfSZ9Wmj0XbiRMg3SH7HiK8kxcIi/ahZS9qY Zgmf3t6yGCX3SriniSx1vgc2LfjhCGo4uZGZ8ZlcKq3IiRCUOn9DUmYl5XEsxnZw==; Received: from [2a02:8108:8980:2478:8cde:aa2c:f324:937e]; authenticated by wp530.webpack.hosteurope.de running ExIM with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) id 1sxoLZ-0005v6-Qm; Mon, 07 Oct 2024 16:04:25 +0200 Message-ID: Date: Mon, 7 Oct 2024 16:04:24 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop To: Vitaly Kuznetsov , Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Kevin Tian , Yan Zhao , Yiwei Zhang , Lai Jiangshan , "Paul E. McKenney" , Josh Triplett , Gerd Hoffmann , Linux kernel regressions list References: <20240309010929.1403984-1-seanjc@google.com> <20240309010929.1403984-6-seanjc@google.com> <877cbyuzdn.fsf@redhat.com> <87iku4ghiw.fsf@redhat.com> From: "Linux regression tracking (Thorsten Leemhuis)" Content-Language: en-US, de-DE Reply-To: Linux regressions mailing list In-Reply-To: <87iku4ghiw.fsf@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-bounce-key: webpack.hosteurope.de;regressions@leemhuis.info;1728309868;6e9e8a64; X-HE-SMSGID: 1sxoLZ-0005v6-Qm On 07.10.24 15:38, Vitaly Kuznetsov wrote: > "Linux regression tracking (Thorsten Leemhuis)" > writes: > >> On 30.08.24 11:35, Vitaly Kuznetsov wrote: >>> Sean Christopherson writes: >>> >>>> Unconditionally honor guest PAT on CPUs that support self-snoop, as >>>> Intel has confirmed that CPUs that support self-snoop always snoop caches >>>> and store buffers. I.e. CPUs with self-snoop maintain cache coherency >>>> even in the presence of aliased memtypes, thus there is no need to trust >>>> the guest behaves and only honor PAT as a last resort, as KVM does today. >>>> >>>> Honoring guest PAT is desirable for use cases where the guest has access >>>> to non-coherent DMA _without_ bouncing through VFIO, e.g. when a virtual >>>> (mediated, for all intents and purposes) GPU is exposed to the guest, along >>>> with buffers that are consumed directly by the physical GPU, i.e. which >>>> can't be proxied by the host to ensure writes from the guest are performed >>>> with the correct memory type for the GPU. >>> >>> Necroposting! >>> >>> Turns out that this change broke "bochs-display" driver in QEMU even >>> when the guest is modern (don't ask me 'who the hell uses bochs for >>> modern guests', it was basically a configuration error :-). E.g: >>> [...] >> >> This regression made it to the list of tracked regressions. It seems >> this thread stalled a while ago. Was this ever fixed? Does not look like >> it, but I might have missed something. Or is this a regression I should >> just ignore for one reason or another? >> > > The regression was addressed in by reverting 377b2f359d1f in 6.11 > > commit 9d70f3fec14421e793ffbc0ec2f739b24e534900 > Author: Paolo Bonzini > Date: Sun Sep 15 02:49:33 2024 -0400 > > Revert "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop" Thx. Sorry, missed that, thx for pointing me towards it. I had looked for things like that, but seems I messed up my lore query. Apologies for the noise! > Also, there's a (pending) DRM patch fixing it from the guest's side: > https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/9388ccf69925223223c87355a417ba39b13a5e8e Great! Ciao, Thorsten P.S.: #regzbot fix: 9d70f3fec14421e793ffbc0ec2f739b24e534900