linux-kernel - Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further

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Message-ID: <alpine.DEB.2.21.1911070843490.1869@nanos.tec.linutronix.de>
Date:   Thu, 7 Nov 2019 08:44:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...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 5/9] x86/ioport: Reduce ioperm impact for sane usage
 further

On Wed, 6 Nov 2019, Linus Torvalds wrote:
> I may read this patch wrong, but from what I can tell, if we really
> have just one process with an io bitmap, we're doing unnecessary
> copies.
> 
> If we really have just one process that has an iobitmap, I think we
> could just keep the bitmap of that process entirely unchanged. Then,
> when we switch away from it, we set the io_bitmap_base to an invalid
> base outside the TSS segment, and when we switch back, we set it back
> to the valid one. No actual bitmap copies at all.
> 
> So I think that rather than the "begin/end offset" games, we should
> perhaps have a "what was the last process that used the IO bitmap for
> this TSS" pointer (and, I think, some sequence counter, so that when
> the process updates its bitmap, it invalidates that case)?
> 
>  Of course, you can do *nboth*, but if we really think that the common
> case is "one special process", then I think the begin/end offset is
> useless, but a "last bitmap process" would be very useful.
> 
> Am I missing something?

No. You are right. I'll have a look at that.

Thanks,

	tglx