On Tue, 2007-03-13 at 08:59 +1100, Rusty Russell wrote:
No, I don't think I will. The PDA concept has gone too far in x86-64 to
be undone. In particular, it's been put in GCC 4.1 for
CONFIG_CC_STACKPROTECTOR, which assumes %gs:40 will give the stack
canary.
For the record: the PDA should never have existed, that's what percpu
vars were supposed to be for. Something went wrong here 8(
%gs is best set to the offset of the local cpu's area from the "master"
per-cpu area, not set to the local cpu area's address. In the former
case, booting with %gs at offset 0 works naturally, in the latter case,
hoops need to be jumped through to make it work. See how much nicer the
x86 code is post pda->percpu conversion.
So, even if we leave the PDA and place the per-cpu area immediately
after it, we still can't use "%gs:var" to access a per-cpu variable: we
need to do a subtract, so why bother using the segment reg?
The ideal solution has always been to use __thread, but no architecture
has yet managed it (I tried for i386, and it quickly caused unbearable
pain). On x86-64 that uses "%fs" on x86-64, not "%gs" as the kernel
does, but I might try that if I feel particularly masochistic soon...
In summary, containing the PDA infection to x86-64 is possible, but
curing that patient is non-trivial 8)
Rusty.
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