From: Brian Gerst <brgerst@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
"the arch/x86 maintainers" <x86@kernel.org>,
Stephen Hemminger <stephen@networkplumber.org>,
Willy Tarreau <w@1wt.eu>, Juergen Gross <jgross@suse.com>,
Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further
Date: Thu, 7 Nov 2019 21:12:38 -0500 [thread overview]
Message-ID: <CAMzpN2hCrcQg_u5sp7WWGjOBv13+ZWtSAecp6bWpT6rsTyo+-Q@mail.gmail.com> (raw)
In-Reply-To: <6cac6943-2f6c-d48a-658e-08b3bf87921a@zytor.com>
On Thu, Nov 7, 2019 at 8:12 PM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On 2019-11-07 13:44, Linus Torvalds wrote:
> > On Thu, Nov 7, 2019 at 1:00 PM Brian Gerst <brgerst@gmail.com> wrote:
> >>
> >> There wouldn't have to be a flush on every task switch.
> >
> > No. But we'd have to flush on any switch that currently does that memcpy.
> >
> > And my point is that a tlb flush (even the single-page case) is likely
> > more expensive than the memcpy.
> >
> >> Going a step further, we could track which task is mapped to the
> >> current cpu like proposed above, and only flush when a different task
> >> needs the IO bitmap, or when the bitmap is being freed on task exit.
> >
> > Well, that's exactly my "track the last task" optimization for copying
> > the thing.
> >
> > IOW, it's the same optimization as avoiding the memcpy.
> >
> > Which I think is likely very effective, but also makes it fairly
> > pointless to then try to be clever..
> >
> > So the basic issue remains that playing VM games has almost
> > universally been slower and more complex than simply not playing VM
> > games. TLB flushes - even invlpg - tends to be pretty slow.
> >
> > Of course, we probably end up invalidating the TLB's anyway, so maybe
> > in this case we don't care. The ioperm bitmap is _technically_
> > per-thread, though, so it should be flushed even if the VM isn't
> > flushed...
> >
>
> One option, probably a lot saner (if we care at all, after all, copying 8K
> really isn't that much, but it might have some impact on real-time processes,
> which is one of the rather few use cases for direct I/O) would be to keep the
> bitmask in a pre-formatted TSS (ioperm being per thread, so no concerns about
> the TSS being in use on another processor), and copy the TSS fields (88 bytes)
> over if and only if the thread has been migrated to a different CPU, then
> switch the TSS rather than switching For the common case (no ioperms) we use
> the standard per-cpu TSS.
>
> That being said, I don't actually know that copying 88 bytes + LTR is any
> cheaper than copying 8K.
I don't think that can work. The TSS has to be at a fixed address in
the cpu_entry_area so that it is visible when running in usermode
(thanks to Meltdown).
--
Brian Gerst
next prev parent reply other threads:[~2019-11-08 2:12 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-06 19:34 [patch 0/9] x86/iopl: Prevent user space from using CLI/STI with iopl(3) Thomas Gleixner
2019-11-06 19:35 ` [patch 1/9] x86/ptrace: Prevent truncation of bitmap size Thomas Gleixner
2019-11-07 7:31 ` Ingo Molnar
2019-11-06 19:35 ` [patch 2/9] x86/process: Unify copy_thread_tls() Thomas Gleixner
2019-11-08 22:31 ` Andy Lutomirski
2019-11-08 23:43 ` Thomas Gleixner
2019-11-10 12:36 ` Thomas Gleixner
2019-11-10 16:56 ` Andy Lutomirski
2019-11-11 8:52 ` Peter Zijlstra
2019-11-06 19:35 ` [patch 3/9] x86/cpu: Unify cpu_init() Thomas Gleixner
2019-11-08 22:34 ` Andy Lutomirski
2019-11-06 19:35 ` [patch 4/9] x86/io: Speedup schedule out of I/O bitmap user Thomas Gleixner
2019-11-07 9:12 ` Peter Zijlstra
2019-11-07 14:04 ` Thomas Gleixner
2019-11-07 14:08 ` Thomas Gleixner
2019-11-08 22:41 ` Andy Lutomirski
2019-11-08 23:45 ` Thomas Gleixner
2019-11-09 3:32 ` Andy Lutomirski
2019-11-10 12:43 ` Thomas Gleixner
2019-11-09 0:24 ` Andy Lutomirski
2019-11-06 19:35 ` [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further Thomas Gleixner
2019-11-07 1:11 ` Linus Torvalds
2019-11-07 7:44 ` Thomas Gleixner
2019-11-07 8:25 ` Ingo Molnar
2019-11-07 9:17 ` Willy Tarreau
2019-11-07 10:00 ` Thomas Gleixner
2019-11-07 10:13 ` Willy Tarreau
2019-11-07 10:19 ` hpa
2019-11-07 10:27 ` Willy Tarreau
2019-11-07 10:50 ` hpa
2019-11-07 12:56 ` Willy Tarreau
2019-11-07 16:45 ` Eric W. Biederman
2019-11-07 16:53 ` Linus Torvalds
2019-11-07 16:57 ` Willy Tarreau
2019-11-10 17:17 ` Andy Lutomirski
2019-11-07 7:37 ` Ingo Molnar
2019-11-07 7:45 ` Thomas Gleixner
2019-11-07 8:16 ` Ingo Molnar
2019-11-07 18:02 ` Thomas Gleixner
2019-11-07 19:24 ` Brian Gerst
2019-11-07 19:54 ` Linus Torvalds
2019-11-07 21:00 ` Brian Gerst
2019-11-07 21:32 ` Thomas Gleixner
2019-11-07 23:20 ` hpa
2019-11-07 21:44 ` Linus Torvalds
2019-11-08 1:12 ` H. Peter Anvin
2019-11-08 2:12 ` Brian Gerst [this message]
2019-11-10 17:21 ` Andy Lutomirski
2019-11-06 19:35 ` [patch 6/9] x86/iopl: Fixup misleading comment Thomas Gleixner
2019-11-06 19:35 ` [patch 7/9] x86/iopl: Restrict iopl() permission scope Thomas Gleixner
2019-11-07 9:09 ` Peter Zijlstra
2019-11-10 17:26 ` Andy Lutomirski
2019-11-10 20:31 ` Thomas Gleixner
2019-11-10 21:05 ` Andy Lutomirski
2019-11-10 21:21 ` Thomas Gleixner
2019-11-06 19:35 ` [patch 8/9] x86/iopl: Remove legacy IOPL option Thomas Gleixner
2019-11-07 6:11 ` Jürgen Groß
2019-11-07 6:26 ` hpa
2019-11-07 16:44 ` Stephen Hemminger
2019-11-07 9:13 ` Peter Zijlstra
2019-11-06 19:35 ` [patch 9/9] selftests/x86/iopl: Verify that CLI/STI result in #GP Thomas Gleixner
2019-11-07 7:28 ` [patch] x86/iopl: Remove unused local variable, update comments in ksys_ioperm() Ingo Molnar
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