From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41455) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XBljZ-0001ll-Mm for qemu-devel@nongnu.org; Mon, 28 Jul 2014 10:17:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XBljP-0001So-QL for qemu-devel@nongnu.org; Mon, 28 Jul 2014 10:17:01 -0400 Received: from e06smtp10.uk.ibm.com ([195.75.94.106]:53616) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XBljP-0001SS-II for qemu-devel@nongnu.org; Mon, 28 Jul 2014 10:16:51 -0400 Received: from /spool/local by e06smtp10.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 28 Jul 2014 15:16:49 +0100 Received: from b06cxnps4075.portsmouth.uk.ibm.com (d06relay12.portsmouth.uk.ibm.com [9.149.109.197]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id 0209C219005F for ; Mon, 28 Jul 2014 15:16:32 +0100 (BST) Received: from d06av10.portsmouth.uk.ibm.com (d06av10.portsmouth.uk.ibm.com [9.149.37.251]) by b06cxnps4075.portsmouth.uk.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id s6SEGkE734406546 for ; Mon, 28 Jul 2014 14:16:47 GMT Received: from d06av10.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av10.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id s6SEGjWt006471 for ; Mon, 28 Jul 2014 08:16:46 -0600 Date: Mon, 28 Jul 2014 16:16:44 +0200 From: David Hildenbrand Message-ID: <20140728161644.00c09b3f@thinkpad-w530> In-Reply-To: <53D654D2.40308@suse.de> References: <1404997839-29038-1-git-send-email-borntraeger@de.ibm.com> <1404997839-29038-5-git-send-email-borntraeger@de.ibm.com> <53D654D2.40308@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH/RFC 4/5] s390x/kvm: test whether a cpu is STOPPED when checking "has_work" List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alexander Graf Cc: linux-s390 , KVM , qemu-devel , Christian Borntraeger , Jens Freimann , Cornelia Huck , Paolo Bonzini > > On 10.07.14 15:10, Christian Borntraeger wrote: > > From: David Hildenbrand > > > > If a cpu is stopped, it must never be allowed to run and no interrupt may wake it > > up. A cpu also has to be unhalted if it is halted and has work to do - this > > scenario wasn't hit in kvm case yet, as only "disabled wait" is processed within > > QEMU. > > > > Signed-off-by: David Hildenbrand > > Reviewed-by: Cornelia Huck > > Reviewed-by: Christian Borntraeger > > Signed-off-by: Christian Borntraeger > > This looks like it's something that generic infrastructure should take > care of, no? How does this work for the other archs? They always get an > interrupt on the transition between !has_work -> has_work. Why don't we > get one for s390x? > > > Alex > > Well, we have the special case on s390 as a CPU that is in the STOPPED or the CHECK STOP state may never run - even if there is an interrupt. It's basically like this CPU has been switched off. Imagine that it is tried to inject an interrupt into a stopped vcpu. It will kick the stopped vcpu and thus lead to a call to "kvm_arch_process_async_events()". We have to deny that this vcpu will ever run as long as it is stopped. It's like a way to "suppress" the interrupt for such a transition you mentioned. Later, another vcpu might decide to turn that vcpu back on (by e.g. sending a SIGP START to that vcpu). I am not sure if such a mechanism/scenario is applicable to any other arch. They all seem to reset the cs->halted flag if they know they are able to run (e.g. due to an interrupt) - they have no such thing as "stopped cpus", only "halted/waiting cpus". David