On Thu, Jan 21, 2021 at 10:28:16AM +0000, Catangiu, Adrian Costin wrote: > On 12/01/2021, 14:49, "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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.
