On Thu, Jan 28, 2021 at 01:58:12PM +0100, Alexander Graf wrote: > 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" <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. > > 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? :) I'd drop mmap at this point. I haven't seen a way to use it that isn't racy. > > 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 > >
