On 15.10.2020 12:41, Jürgen Groß wrote: > On 15.10.20 12:09, Jan Beulich wrote: >> On 15.10.2020 09:58, Jürgen Groß wrote: >>> After a short discussion on IRC yesterday I promised to send a mail >>> how I think we could get rid of creating dynamic links especially >>> for header files in the Xen build process. >>> >>> This will require some restructuring, the amount will depend on the >>> selected way to proceed: >>> >>> - avoid links completely, requires more restructuring >>> - avoid only dynamically created links, i.e. allowing some static >>> links which are committed to git >> >> While I like the latter better, I'd like to point out that not all >> file systems support symlinks, and hence the repo then couldn't be >> stored on (or the tarball expanded onto) such a file system. Note >> that this may be just for viewing purposes - I do this typically at >> home -, i.e. there's no resulting limitation from the build process >> needing symlinks. Similarly, once we fully support out of tree >> builds, there wouldn't be any restriction from this as long as just >> the build tree is placed on a capable file system. >> >> As a result I'd like to propose variant 2´: Reduce the number of >> dynamically created symlinks to a minimum. This said, I have to >> admit that I haven't really understood yet why symlinks are bad. >> They exist for exactly such purposes, I would think. > > Not the symlinks as such, but the dynamically created ones seem to be > a problem, as we stumble upon those again and again.
Well, the machinery to get them put in place needs to be fixed (and adjustments / additions be done more carefully). Taking together with what Andrew has said, option 2´ would move us in the same direction then. >>> The difference between both variants is affecting the public headers >>> in xen/include/public/: avoiding even static links would require to >>> add another directory or to move those headers to another place in the >>> tree (either use xen/include/public/xen/, or some other path */xen), >>> leading to the need to change all #include statements in the hypervisor >>> using <public/...> today. >>> >>> The need for the path to have "xen/" is due to the Xen library headers >>> (which are installed on user's machines) are including the public >>> hypervisor headers via "#include <xen/...>" and we can't change that >>> scheme. A static link can avoid this problem via a different path, but >>> without any link we can't do that. >>> >>> Apart from that decision, lets look which links are created today for >>> accessing the header files (I'll assume my series putting the library >>> headers to tools/include will be taken, so those links being created >>> in staging today are not mentioned) and what can be done to avoid them: >>> >>> - xen/include/asm -> xen/include/asm-<arch>: >>> Move all headers from xen/include/asm-<arch> to >>> xen/arch/<arch>/include/asm and add that path via "-I" flag to CFLAGS. >>> This has the other nice advantages that most architecture specific >>> files are now in xen/arch (apart from the public headers) and that we >>> can even add generic fallback headers in xen/include/asm in case an >>> arch doesn't need a specific header file. >> >> Iirc Andrew suggested years ago that we follow Linux in this regard >> (and XTF already does). My only concern here is the churn this will >> cause for backports. > > Changing a directory name in a patch isn't that hard, IMO. It's not hard at all, no, but it still takes some of the most precious resource we have: time. >>> - tools/include/xen/foreign -> tools/include/xen-foreign: >>> Get rid of tools/include/xen-foreign and generate the headers directly >>> in xen/include/public/foreign instead. >> >> Except that conceptually building in tools/ would better not alter >> the xen/ subtree in any way. > > I meant to generate the headers from the hypervisor build instead. This would make the tools/ build dependent upon xen/ having got built first aiui, which I think we want to avoid. >>> - tools/include/xen/lib/<arch>/* -> xen/include/xen/lib/<arch>/*: >>> Move xen/include/xen/lib/<arch> to xen/include/tools/lib/<arch> and >>> add "-Ixen/include/tools" to the CFLAGS of tools. >> >> Why not -Ixen/include/xen without any movement? Perhaps because > > This would open up most of the hypervisor private headers to be > easily includable by tools. Without the xen/ prefix, yes. But if someone wants to violate the naming scheme to get at them, adding a suitable number of ../ will also work as soon as symlinks aren't being used, or symlinks of full directories are used instead of ones referencing individual files. Jan
