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[79.177.39.148]) by smtp.gmail.com with ESMTPSA id z184sm2618398wmg.7.2021.01.27.04.47.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 Jan 2021 04:47:21 -0800 (PST) Date: Wed, 27 Jan 2021 07:47:17 -0500 From: "Michael S. Tsirkin" To: "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" , "Graf (AWS), Alexander" , "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" Subject: Re: [PATCH v4 0/2] System Generation ID driver and VMGENID backend Message-ID: <20210127074549-mutt-send-email-mst@kernel.org> References: <1610453760-13812-1-git-send-email-acatan@amazon.com> <20210112074658-mutt-send-email-mst@kernel.org> <9952EF0C-CD1D-4EDB-BAB8-21F72C0BF90D@amazon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9952EF0C-CD1D-4EDB-BAB8-21F72C0BF90D@amazon.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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 exposes 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 read() > > which blocks until the SysGenId changes. Following an update, read() > > 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. Alternatively, > > 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 offset, > 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 importantly 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. > Thanks, > Adrian. > > > > > Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.