From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp-fw-6001.amazon.com ([52.95.48.154]:62208 "EHLO smtp-fw-6001.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231932AbhA1M7R (ORCPT ); Thu, 28 Jan 2021 07:59:17 -0500 Subject: Re: [PATCH v4 0/2] System Generation ID driver and VMGENID backend References: <1610453760-13812-1-git-send-email-acatan@amazon.com> <20210112074658-mutt-send-email-mst@kernel.org> <9952EF0C-CD1D-4EDB-BAB8-21F72C0BF90D@amazon.com> <20210127074549-mutt-send-email-mst@kernel.org> From: Alexander Graf Message-ID: <7bcd1cf3-d055-db46-95ea-5c023df2f184@amazon.de> Date: Thu, 28 Jan 2021 13:58:12 +0100 MIME-Version: 1.0 In-Reply-To: <20210127074549-mutt-send-email-mst@kernel.org> Content-Language: en-US Content-Type: text/plain; charset="windows-1252"; format="flowed" Content-Transfer-Encoding: quoted-printable List-ID: To: "Michael S. Tsirkin" , "Catangiu, Adrian Costin" Cc: "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "qemu-devel@nongnu.org" , "kvm@vger.kernel.org" , "linux-s390@vger.kernel.org" , "gregkh@linuxfoundation.org" , "arnd@arndb.de" , "ebiederm@xmission.com" , "rppt@kernel.org" , "0x7f454c46@gmail.com" <0x7f454c46@gmail.com>, "borntraeger@de.ibm.com" , "Jason@zx2c4.com" , "jannh@google.com" , "w@1wt.eu" , "MacCarthaigh, Colm" , "luto@kernel.org" , "tytso@mit.edu" , "ebiggers@kernel.org" , "Woodhouse, David" , "bonzini@gnu.org" , "Singh, Balbir" , "Weiss, Radu" , "corbet@lwn.net" , "mhocko@kernel.org" , "rafael@kernel.org" , "pavel@ucw.cz" , "mpe@ellerman.id.au" , "areber@redhat.com" , "ovzxemul@gmail.com" , "avagin@gmail.com" , "ptikhomirov@virtuozzo.com" , "gil@azul.com" , "asmehra@redhat.com" , "dgunigun@redhat.com" , "vijaysun@ca.ibm.com" , "oridgar@gmail.com" , "ghammer@redhat.com" Hey Michael! On 27.01.21 13:47, Michael S. Tsirkin wrote: > = > On Thu, Jan 21, 2021 at 10:28:16AM +0000, Catangiu, Adrian Costin wrote: >> On 12/01/2021, 14:49, "Michael S. Tsirkin" wrote: >> >> On Tue, Jan 12, 2021 at 02:15:58PM +0200, Adrian Catangiu wrote: >> > The first patch in the set implements a device driver which expos= es a >> > read-only device /dev/sysgenid to userspace, which contains a >> > monotonically increasing u32 generation counter. Libraries and >> > applications are expected to open() the device, and then call rea= d() >> > which blocks until the SysGenId changes. Following an update, rea= d() >> > calls no longer block until the application acknowledges the new >> > SysGenId by write()ing it back to the device. Non-blocking read()= calls >> > return EAGAIN when there is no new SysGenId available. Alternativ= ely, >> > libraries can mmap() the device to get a single shared page which >> > contains the latest SysGenId at offset 0. >> >> Looking at some specifications, the gen ID might actually be located >> at an arbitrary address. How about instead of hard-coding the offse= t, >> we expose it e.g. in sysfs? >> >> The functionality is split between SysGenID which exposes an internal u32 >> counter to userspace, and an (optional) VmGenID backend which drives >> SysGenID generation changes based on hw vmgenid updates. >> >> The hw UUID you're referring to (vmgenid) is not mmap-ed to userspace or >> otherwise exposed to userspace. It is only used internally by the vmgenid >> driver to find out about VM generation changes and drive the more generic >> SysGenID. >> >> The SysGenID u32 monotonic increasing counter is the one that is mmaped = to >> userspace, but it is a software counter. I don't see any value in using = a dynamic >> offset in the mmaped page. Offset 0 is fast and easy and most importantl= y it is >> static so no need to dynamically calculate or find it at runtime. > = > Well you are burning a whole page on it, using an offset the page > can be shared with other functionality. Currently, the SysGenID lives is one page owned by Linux that we share = out to multiple user space clients. So yes, we burn a single page of the = system here. If we put more data in that same page, what data would you put there? = Random other bits from other subsystems? At that point, we'd be = reinventing vdso all over again, no? Probably with the same problems. Which gets me to the second alternative: Reuse VDSO. The problem there = is that the VDSO is an extremely architecture specific mechanism. Any = new architecture we'd want to support would need multiple layers of = changes in multiple layers of both kernel and libc. I'd like to avoid = that if we can :). So that leaves us with either wasting a page per system or not having an = mmap() interface in the first place. The reason we have the mmap() interface is that it's be easier to = consume for libraries, that are not hooked into the main event loop. So, uh, what are you suggesting? :) Alex Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879