From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751224AbdFBH0H (ORCPT ); Fri, 2 Jun 2017 03:26:07 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:33510 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751126AbdFBH0F (ORCPT ); Fri, 2 Jun 2017 03:26:05 -0400 Subject: Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc_pgste From: Christian Borntraeger To: Martin Schwidefsky Cc: Heiko Carstens , David Hildenbrand , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Huth References: <20170529163202.13077-1-david@redhat.com> <20170601124651.3e7969ab@mschwideX1> <20170602070210.GA4221@osiris> <20170602091638.5583cfb2@mschwideX1> <787556bd-2c20-48e8-edd4-f4c5c5a5eaaf@de.ibm.com> Date: Fri, 2 Jun 2017 09:25:54 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0 MIME-Version: 1.0 In-Reply-To: <787556bd-2c20-48e8-edd4-f4c5c5a5eaaf@de.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-IE Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 17060207-0024-0000-0000-0000168EC1FA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007157; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00868954; UDB=6.00431946; IPR=6.00648919; BA=6.00005393; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015675; XFM=3.00000015; UTC=2017-06-02 07:26:00 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17060207-0025-0000-0000-00004B30795E Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-06-02_04:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1706020140 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/02/2017 09:18 AM, Christian Borntraeger wrote: > On 06/02/2017 09:16 AM, Martin Schwidefsky wrote: >> On Fri, 2 Jun 2017 09:13:03 +0200 >> Christian Borntraeger wrote: >> >>> On 06/02/2017 09:02 AM, Heiko Carstens wrote: >>>> On Thu, Jun 01, 2017 at 12:46:51PM +0200, Martin Schwidefsky wrote: >>>>>> Unfortunately, converting all page tables to 4k pgste page tables is >>>>>> not possible without provoking various race conditions. >>>>> >>>>> That is one approach we tried and was found to be buggy. The point is that >>>>> you are not allowed to reallocate a page table while a VMA exists that is >>>>> in the address range of that page table. >>>>> >>>>> Another approach we tried is to use an ELF flag on the qemu executable. >>>>> That does not work either because fs/exec.c allocates and populates the >>>>> new mm struct for the argument pages before fs/binfmt_elf.c comes into >>>>> play. >>>> >>>> How about if you would fail the system call within arch_check_elf() if you >>>> detect that the binary requires pgstes (as indicated by elf flags) and then >>>> restart the system call? >>>> >>>> That is: arch_check_elf() e.g. would set a thread flag that future mm's >>>> should be allocated with pgstes. Then do_execve() would cleanup everything >>>> and return to entry.S. Upon return to userspace we detect this condition >>>> and simply restart the system call, similar to signals vs -ERESTARTSYS. >>>> >>>> That would make do_execve() cleanup everything and upon reentering it would >>>> allocate an mm with the pgste flag set. >>>> >>>> Maybe this is a bit over-simplified, but might work. >>>> >>>> At least I also don't like the next "hack", that is specifically designed >>>> to only work with how QEMU is currently implemented. It might break with >>>> future QEMU changes or the next user space implementation that drives the >>>> kvm interface, but is doing everything differently. >>>> Let's look for a "clean" solution that will always work. We had too many >>>> hacks for this problem and *all* of them were broken. >>> >>> >>> The more I think about it, dropping 2k page tables and always allocate a full >>> page would simplify pgalloc. As far I can see this would also get rid of >>> the &mm->context.pgtable_lock. >> >> And it would waste twice the amount of memory for page tables. NAK. > > Yes and we spend the same amount of memory TODAY, because every distro on the > planet that uses KVM has sysctl.allocate_pgste set. Maybe todays approach might be still the best. (if qemu is installed, its all 4k, if not its all 2k)