From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pdx-out-006.esa.us-west-2.outbound.mail-perimeter.amazon.com (pdx-out-006.esa.us-west-2.outbound.mail-perimeter.amazon.com [52.26.1.71]) (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 9FCA03D3319 for ; Tue, 10 Mar 2026 05:58:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=52.26.1.71 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773122308; cv=none; b=LLavCUKHdcTtH4tK9jaY0EyWKrPATDdDqLXuXES6poqBitF7AQseFg8vpbAfBszP+VKS2Roy6LCdAxutMFu+tWfqwBRsegWcfsmhAdxXYc7PYBvfzaKJl4ihXT2UvucT2qH6Y9Z4wQLe9hhKZ8gEdhOwdupXPIflmiTtTVu/G2o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773122308; c=relaxed/simple; bh=YwbmprIkDtutINfk3sBxaUCdm/OmSlnPX5jH2QN1F0Y=; h=From:To:CC:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=aWgq3TBGftS/DcWPIhge8ol36tjKzgH+5EOHlPPlj3zfq3e2m1+EDSj0SlDIVuA4jo3cx/RRBwzUdfSrnW8MaXz9O7kpt/Kdpjdz73ljSB5vBgRFJJqqTMH6GqsjPPvKl19hjJ1WZDHDeUcEpFldIE2YfO2LHVpAkyhnSejZq0w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.com; spf=pass smtp.mailfrom=amazon.co.uk; dkim=pass (2048-bit key) header.d=amazon.com header.i=@amazon.com header.b=c7ps2RiT; arc=none smtp.client-ip=52.26.1.71 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=amazon.co.uk Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=amazon.com header.i=@amazon.com header.b="c7ps2RiT" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazoncorp2; t=1773122301; x=1804658301; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=nt2g2G+5DYfHQuv+2HMCz3ZelNBjKrMJvT2XXgxuY1E=; b=c7ps2RiTiggnP08GgoDKUm948670GwAfb2dE6yy7U8jFwlo/p0c1Jc+8 uYraBS8/gKeLA1b/AWZkJhTzODtmsKh2dMLZz46yBfCSeFN77B+qmO0rX iYZIGyNr5qgijdyvLMnzUZdIJEEOe3RbQBEF/8OpfBbpLm4gW/l2XhR4L i5Od0sFoIo7yjFhA+YCeU2/l72HU46LDKRTeNrvzfegzG+zvNyl5FX57+ 02QVf+cnm/Jcr7wz3++RXloAxgCd2hk590E4M3LWQZUw/O/ULIVa74omi iHnSwR8P4koaBi5p8zEu9xKxWxQ2mkWNA/y4PdtFP++feJ2RDXQlnwSf8 A==; X-CSE-ConnectionGUID: g5GgTMBBSFqBXQHIs+IinQ== X-CSE-MsgGUID: 6w3mohR+Q42GMyTInTiIEg== X-IronPort-AV: E=Sophos;i="6.23,111,1770595200"; d="scan'208";a="14688221" Received: from ip-10-5-9-48.us-west-2.compute.internal (HELO smtpout.naws.us-west-2.prod.farcaster.email.amazon.dev) ([10.5.9.48]) by internal-pdx-out-006.esa.us-west-2.outbound.mail-perimeter.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Mar 2026 05:58:18 +0000 Received: from EX19MTAUWB001.ant.amazon.com [205.251.233.51:30114] by smtpin.naws.us-west-2.prod.farcaster.email.amazon.dev [10.0.52.102:2525] with esmtp (Farcaster) id 5a5a57e9-3fa5-470f-b761-4fae4911677a; Tue, 10 Mar 2026 05:58:18 +0000 (UTC) X-Farcaster-Flow-ID: 5a5a57e9-3fa5-470f-b761-4fae4911677a Received: from EX19D001UWA001.ant.amazon.com (10.13.138.214) by EX19MTAUWB001.ant.amazon.com (10.250.64.248) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.37; Tue, 10 Mar 2026 05:58:16 +0000 Received: from dev-dsk-itazur-1b-11e7fc0f.eu-west-1.amazon.com (172.19.66.53) by EX19D001UWA001.ant.amazon.com (10.13.138.214) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.37; Tue, 10 Mar 2026 05:58:14 +0000 From: Takahiro Itazuri To: CC: , , , , , , , , , , , Subject: Re: Re: [RFC PATCH v2 1/7] KVM: x86: Avoid silent kvm-clock activation failures Date: Tue, 10 Mar 2026 05:58:09 +0000 Message-ID: <20260310055809.2690-1-itazur@amazon.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain X-ClientProxiedBy: EX19D036UWC003.ant.amazon.com (10.13.139.214) To EX19D001UWA001.ant.amazon.com (10.13.138.214) On Thu, 5 Mar 2026 09:50:30 -0800, Sean Christopherson wrote: > On Thu, Feb 26, 2026, Takahiro Itazuri wrote: > > kvm_write_system_time() previously ignored the return value of > > kvm_gpc_activate(). As a result, kvm-clock activation could fail > > silently, making debugging harder. > >=20 > > Propagate the return value so that the MSR write fail properly instead > > of continuing silently. >=20 > Hrm. For better or worse, KVM's ABI when it comes to PV stuff is to sile= ntly > ignore failures. I 100% agree it makes debugging painful, but it's unfor= tunately > also "safer" in many cases, e.g. often results in degraded behavior versu= s flat > out crashing the guest. >=20 > The other wrinkle is that success isn't actually guaranteed, because the = actual > writes don't happen until KVM_RUN via kvm_guest_time_update(), i.e. only = failing > in _some_ cases creates a weird ABI. >=20 > And most importantly, this would be a breaking change in guest- and user-= visible > behavior. So while I agree silently failing is ugly, all things consider= ed I > think it's the least awful choice here :-/ Fair point. I'll drop this change in the next version since I spotted a compile error in another patch and I need to resend anyway.