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

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Date:   Thu, 7 Nov 2019 15:08:44 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 4/9] x86/io: Speedup schedule out of I/O bitmap user

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