Jeremy Fitzhardinge wrote:
I'm implementing a more efficient version of the Xen iret paravirt_op,
so that it can use the real iret instruction where possible. I really
need to get access to per-cpu variables, so I can set the event mask
state in the vcpu_info structure, but unfortunately at the point where
INTERRUPT_RETURN is used in entry.S, the usermode %fs has already been
restored.
How would you feel if we changed paravirt_ops.iret to make it also
responsible for restoring %fs?
In other words, change RESTORE_REGS to skip %fs, and then native_iret
would look like:
1: popl %fs
iret
with the normal exception stuff. Fortunately, %fs is already the first
thing to be saved and last to be restored, so there's no major
rearrangements.
Ideally I'd also like a register to play with, but that would require
rearranging pt_regs, which is a bit tricky.
Strongly nacked. If you need %fs and a free register, just push / pop
them yourself. We use push / pop to get %eax free for IRET in the VMI ROM.
A change in IRET call convention is performance critical, and has severe
negative effects on VMI, since we then need to wrap the IRET entry point
in the ROM with more code in Linux - so iret is a jump to fs restore,
call to ROM gobbledygook that misaligns the call/return stack and rots
performance in some hitherto unkown case - perhaps on native.
There is absolutely no argument that re-arranging entry.S like this is
nightmarish and makes the code near unmaintainable without carefully
considering the effects on paravirt-ops hypervisors.
If you really absolutely must avoid reloading %fs for your own
performance, I can suggest several alternative schemes that, while more
complex, might also make native exception handling faster, thus
justifying the hack-to-bitty-pieces of entry.S
Zach
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