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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 10 Feb 2020 18:51:51 -0000 Received: from d06av21.portsmouth.uk.ibm.com (d06av21.portsmouth.uk.ibm.com [9.149.105.232]) by b06avi18878370.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 01AIpn3f34406710 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 10 Feb 2020 18:51:49 GMT Received: from d06av21.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7408F5204E; Mon, 10 Feb 2020 18:51:49 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.145.7.195]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTP id 997E852050; Mon, 10 Feb 2020 18:51:48 +0000 (GMT) Subject: Re: [PATCH 01/35] mm:gup/writeback: add callbacks for inaccessible pages To: David Hildenbrand , Janosch Frank Cc: KVM , Cornelia Huck , Thomas Huth , Ulrich Weigand , Claudio Imbrenda , Andrea Arcangeli , linux-s390 , Michael Mueller , Vasily Gorbik , linux-mm@kvack.org, Andrew Morton , Michal Hocko References: <20200207113958.7320-1-borntraeger@de.ibm.com> <20200207113958.7320-2-borntraeger@de.ibm.com> From: Christian Borntraeger Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 20021018-0016-0000-0000-000002E59074 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 20021018-0017-0000-0000-000033488395 Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138,18.0.572 definitions=2020-02-10_06:2020-02-10,2020-02-10 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 mlxlogscore=999 spamscore=0 clxscore=1015 bulkscore=0 phishscore=0 mlxscore=0 lowpriorityscore=0 suspectscore=0 adultscore=0 impostorscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2002100138 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 10.02.20 19:43, David Hildenbrand wrote: > On 10.02.20 19:28, Christian Borntraeger wrote: >> >> >> On 10.02.20 19:17, David Hildenbrand wrote: >>> On 07.02.20 12:39, Christian Borntraeger wrote: >>>> From: Claudio Imbrenda >>>> >>>> With the introduction of protected KVM guests on s390 there is now a >>>> concept of inaccessible pages. These pages need to be made accessible >>>> before the host can access them. >>>> >>>> While cpu accesses will trigger a fault that can be resolved, I/O >>>> accesses will just fail. We need to add a callback into architecture >>>> code for places that will do I/O, namely when writeback is started or >>>> when a page reference is taken. >>> >>> My question would be: What guarantees that the page will stay accessible >>> (for I/O)? IIRC, pages can be converted back to secure/inaccessible >>> whenever the guest wants to access them. How will that be dealt with? >> >> Yes, in patch 5 we do use the page lock, PageWriteBack and page_ref_freeze >> to only make the page secure again if no I/O is going to be started or >> still running. >> >> We have minimized the common code impact (just these 3 callbacks) so that >> architecture code can do the right thing. > > So the magic is > > +static int expected_page_refs(struct page *page) > +{ > + int res; > + > + res = page_mapcount(page); > + if (PageSwapCache(page)) > + res++; > + else if (page_mapping(page)) { > + res++; > + if (page_has_private(page)) > + res++; > + } > + return res; > +} > [...] > +static int make_secure_pte(pte_t *ptep, unsigned long addr, void *data) > [...] > + if (PageWriteback(page)) > + return -EAGAIN; > + expected = expected_page_refs(page); > + if (!page_ref_freeze(page, expected)) > + return -EBUSY; > [...] > + rc = uv_call(0, (u64)params->uvcb); > + page_ref_unfreeze(page, expected); > > As long as a page is does not have the expected refcount, it cannot be > convert to secure and not used by the guest. > > I assume this implies, that if a guest page is pinned somewhere (e.g., > in KVM), it won't be usable by the guest. Yes, but you can always exit QEMU nothing will "block". You you have a permanent SIE exit. This is something that should not happen for QEMU/KVM and the expected refcount logic can be found in many common code places. > > Please add all these details to the patch description. I think they are > crucial to understand how this is expected to work and to be used. Makes sense. Will add more explanation to patch 5. Ulrich also had some idea how to simplify patch 5 in some places.