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

Thomas Gleixner
2019-11-07T14:08:50Z

On Thu, 7 Nov 2019, Thomas Gleixner wrote:
> On Thu, 7 Nov 2019, Peter Zijlstra wrote:
> > 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.
>
> Just that I can't add the storage to tss_struct due to the VMX insanity of
> setting TSS limit hard to 0x67 on vmexit instead of restoring the host
> value.

Well, I can. The build bugon in vmx.c is just bogus.

> Did I say that already that virt creates more problems than it solves?

Still stands.

tglx