Theodore Y. Ts'o wrote on Fri, Dec 28, 2018: > On Sat, Dec 29, 2018 at 03:37:21AM +0100, Dominique Martinet wrote: > > > Are there going to be cases where a process or a thread will sometimes > > > want the 64-bit interface, and sometimes want the 32-bit interface? > > > Or is it always going to be one or the other? I wonder if we could > > > simply add a new flag to the process personality(2) flags. > > > > That would likely work for qemu user, but the qemu system+9p case is > > going to be more painful.. > > More precisely, the 9p protocol does not plan for anything other than > > 64bit offset so if the vfs needs to hand out a 32bit offset we'll need > > to make a correspondance table between the 32bit offsets we hand off and > > the 64bit ones to use; unless some flag can be passed at lopen to tell > > the server to always hand out 32bit offsets for this directory... And if > > we do that then 9p servers will need a way to use both APIs in parallel > > for both types of directories. > > How about if we add a fcntl(2) mediated flag, which is tied to a > struct file? Would that be more or less painful for 9p and qemu > system+9p? Hmm. 9P2000.L doesn't have anything akin to fcntl either, the only two obvious places where we could pass a flag is lopen (which already handles a bunch of linux-specific flags, e.g. passing O_LARGEFILE O_NOATIME etc will just forward these through for qemu/diod at least), or adding a new parameter to the 9p readdir. The former would let us get away without modifying the protocol as servers will just ignore flags they don't handle on implementations I checked, so it'd definitely be the least effort choice from what I can tell. On the other hand a fcntl would solve the server-side problem, it'd allow the server to request appropriately-sized offsets per fd, so it's a good start; we "just" need to figure how to translate that on the wire. -- Dominique Martinet | Asmadeus
