Re: [patch 4/9] x86/io: Speedup schedule out of I/O bitmap user

[Peter Zijlstra]

On Wed, Nov 06, 2019 at 08:35:03PM +0100, Thomas Gleixner wrote:
> There is no requirement to update the TSS I/O bitmap when a thread using it is
> scheduled out and the incoming thread does not use it.
> 
> For the permission check based on the TSS I/O bitmap the CPU calculates the memory
> location of the I/O bitmap by the address of the TSS and the io_bitmap_base member
> of the tss_struct. The easiest way to invalidate the I/O bitmap is to switch the
> offset to an address outside of the TSS limit.
> 
> If an I/O instruction is issued from user space the TSS limit causes #GP to be
> raised in the same was as valid I/O bitmap with all bits set to 1 would do.
> 
> This removes the extra work when an I/O bitmap using task is scheduled out
> and puts the burden on the rare I/O bitmap users when they are scheduled
> in.

This also nicely aligns with that the context switch time is accounted
to the next task. So by doing the expensive part on switch-in gets it
all accounted to the task that caused it.