From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH] KVM: s390: remove delayed reallocation of page tables
 for KVM
Date: Thu, 23 Apr 2015 13:43:16 +0200
Message-ID: <5538DAD4.4060505@de.ibm.com>
References: <1429787297-9292-1-git-send-email-borntraeger@de.ibm.com> <1429787297-9292-2-git-send-email-borntraeger@de.ibm.com> <2E97DEE6-47EA-484A-9F02-9F031DCA8F36@suse.de>
Mime-Version: 1.0
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Cc: Paolo Bonzini <pbonzini@redhat.com>, KVM <kvm@vger.kernel.org>,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	Jens Freimann <jfrei@linux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Alexander Graf <agraf@suse.de>
Return-path: <kvm-owner@vger.kernel.org>
Received: from e06smtp14.uk.ibm.com ([195.75.94.110]:43893 "EHLO
	e06smtp14.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751602AbbDWLnV (ORCPT <rfc822;kvm@vger.kernel.org>);
	Thu, 23 Apr 2015 07:43:21 -0400
Received: from /spool/local
	by e06smtp14.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted
	for <kvm@vger.kernel.org> from <borntraeger@de.ibm.com>;
	Thu, 23 Apr 2015 12:43:19 +0100
Received: from b06cxnps4075.portsmouth.uk.ibm.com (d06relay12.portsmouth.uk.ibm.com [9.149.109.197])
	by d06dlp03.portsmouth.uk.ibm.com (Postfix) with ESMTP id D61B41B08061
	for <kvm@vger.kernel.org>; Thu, 23 Apr 2015 12:43:54 +0100 (BST)
Received: from d06av08.portsmouth.uk.ibm.com (d06av08.portsmouth.uk.ibm.com [9.149.37.249])
	by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t3NBhHb565667312
	for <kvm@vger.kernel.org>; Thu, 23 Apr 2015 11:43:17 GMT
Received: from d06av08.portsmouth.uk.ibm.com (localhost [127.0.0.1])
	by d06av08.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t3NBhGcx025997
	for <kvm@vger.kernel.org>; Thu, 23 Apr 2015 05:43:17 -0600
In-Reply-To: <2E97DEE6-47EA-484A-9F02-9F031DCA8F36@suse.de>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

Am 23.04.2015 um 13:37 schrieb Alexander Graf:
> 
> 
>> Am 23.04.2015 um 13:08 schrieb Christian Borntraeger <borntraeger@de.ibm.com>:
>>
>> From: Martin Schwidefsky <schwidefsky@de.ibm.com>
>>
>> Replacing a 2K page table with a 4K page table while a VMA is active
>> for the affected memory region is fundamentally broken. Rip out the
>> page table reallocation code and replace it with a simple system
>> control 'vm.allocate_pgste'. If the system control is set the page
>> tables for all processes are allocated as full 4K pages, even for
>> processes that do not need it.
>>
>> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> 
> Couldn't you make this a hidden kconfig option that gets automatically selected when kvm is enabled? Or is there a non-kvm case that needs it too?

For things like RHEV the default could certainly be "enabled", but for normal
distros like SLES/RHEL, the idea was to NOT enable that by default, as the non-KVM
case is more common and might suffer from the additional memory consumption of
the page tables. (big databases come to mind)

We could think about having rpms like kvm to provide a sysctl file that sets it if we
want to minimize the impact. Other ideas?

Christian