Making spl7 just cli/sti?

[Samuel Thibault]

Hello,

I've dug quite a bit why gnumach on kvm on kvm is slugguish.  What
happens is that spl7 (the scheduler level for interrupts) is using a
PIC mask to avoid reentrancy.  That is fine on real hardware, but in
a virtualized environment, this is very expensive since it'll need
a VMEXIT, then simulation in the host.  It happens than gnumach is
sometimes doing it a couple hundred thousand times a second!  Attached
patch2 allows to measure this, I do get 200000 during device creation at
debian installer boot.

The attached patch fixes this by making spl.S use just cli/sti for spl7;
they can be emulated fine without any VMEXIT.  This lowers the number of
PIC mask changes to a few hundreds or thousands only, and getting the
gnumach on kvm on kvm case way faster.

Does anybody with a bit of background with spl (I guess Richard?) see
any red flag here?

Samuel

patch
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patch2
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