Messages in this thread | | | Date | Thu, 7 Nov 2019 10:12:31 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch 4/9] x86/io: Speedup schedule out of I/O bitmap user |
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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.
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