On 06/05/2018 08:36 AM, H. Peter Anvin wrote: > On 06/04/18 20:57, Mika Penttilä wrote: >> >> This won't work on X86-32 because it actually uses the segment limit with >> fs: access. So there >> is a reason why the lsl based method is X86-64 only. >> > > <thinks out loud> > > Why does that matter in any shape, way, or form? The LSL instruction > doesn't touch any of the segment registers, it just uses a segment > selector number. > > <looks at code> > > I see... we have a VERY unfortunate name collision: the x86-64 > GDT_ENTRY_PERC_PU and the i386 GDT_ENTRY_PERCPU are in fact two > completely different things, with the latter being the actual percpu > offset used by the kernel. > > So yes, this patch is wrong, because the naming of the x86-64 segment is > insane especially in the proximity of the -- it should be something > like GDT_ENTRY_CPU_NUMBER. > > Unfortunately we probably can't use the same GDT entry on x86-32 and > x86-64, because entry 15 (selector 0x7b) is USER_DS, which is something > we really don't want to screw with. This means i386 programs that > execute LSL directly for whatever reason will have to understand the > difference, but most of the other segment numbers differ as well, > including user space %cs (USER_CS/USER32_CS) and %ds/%es/%ss (USER_DS). > Perhaps we could bump down segments 23-28 by one and put it as 23, that > way the CPU_NUMBER segment would always be at %ss+80 for the default > (flat, initial) user space %ss. (We want to specify using %ss rather > than %ds, because it is less likely to be changed and because 64 bits, > too, have %ss defined, but not %ds.) > > <action item> > > Rename the x86-64 segment to CPU_NUMBER, fixing the naming conflict. > Add 1 to GDT entry numbers 23-28 for i386 (all of these are > kernel-internal segments and so have no impact on user space). > Add i386 CPU_NUMBER equivalent to x86-64 at GDT entry 23. > Document the above relationship between segments. > > OK, everyone? > > -hpa >
Yes GDT_ENTRY_PER_CPU and GDT_ENTRY_PERCPU meaning two totally different things is really confusing, the proposal seems ok to me! --Mika