linux-kernel - Re: [patch V2 09/16] x86/ioperm: Move TSS bitmap update to exit to user work

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Date:   Tue, 12 Nov 2019 08:16:19 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V2 09/16] x86/ioperm: Move TSS bitmap update to exit to
 user work

On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> There is no point to update the TSS bitmap for tasks which use I/O bitmaps
> on every context switch. It's enough to update it right before exiting to
> user space.
>

+
> +static inline void switch_to_bitmap(unsigned long tifp)
> +{
> +       /*
> +        * Invalidate I/O bitmap if the previous task used it. If the next
> +        * task has an I/O bitmap it will handle it on exit to user mode.
> +        */
> +       if (tifp & _TIF_IO_BITMAP)
> +               tss_invalidate_io_bitmap(this_cpu_ptr(&cpu_tss_rw));
> +}

Shouldn't you be invalidating the io bitmap if the *next* task doesn't
use?  Or is the rule that, when a non-io-bitmap-using task is running,
even in kernel mode, the io bitmap is always invalid.

As it stands, you need exit_thread() to invalidate the bitmap.  I
assume it does, but I can't easily see it in the middle of the series
like this.

IOW your code might be fine, but it could at least use some comments
in appropriate places (exit_to_usermode_loop()?) that we guarantee
that, if the bit is *clear*, then the TSS has the io bitmap marked
invalid.  And add an assertion under CONFIG_DEBUG_ENTRY.

Also, do you need to update EXIT_TO_USERMODE_LOOP_FLAGS?