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

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Message-ID: <20191107081635.GE30739@gmail.com>
Date:   Thu, 7 Nov 2019 09:16:35 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 5/9] x86/ioport: Reduce ioperm impact for sane usage
 further


* Thomas Gleixner <tglx@...utronix.de> wrote:

> +	/* Update the bitmap */
> +	if (turn_on) {
> +		bitmap_clear(bitmap, from, num);
> +	} else {
> +		bitmap_set(bitmap, from, num);
> +	}
> +
> +	/* Get the new range */
> +	first = find_first_zero_bit(bitmap, IO_BITMAP_BITS);
> +
> +	for (last = next = first; next < IO_BITMAP_BITS; last = next) {
> +		/* Find the next set bit and update last */
> +		next = find_next_bit(bitmap, IO_BITMAP_BITS, last);
> +		last = next - 1;
> +		if (next == IO_BITMAP_BITS)
> +			break;
> +		/* Find the next zero bit and continue searching */
> +		next = find_next_zero_bit(bitmap, IO_BITMAP_BITS, next);
> +	}
> +
> +	/* Calculate the byte boundaries for the updated region */
> +	copy_start = from / 8;
> +	copy_len = (round_up(from + num, 8) / 8) - copy_start;

This might seem like a small detail, but since we do the range tracking 
and copying at byte granularity anyway, why not do the zero range search 
at byte granularity as well?

I bet it's faster and simpler as well than the bit-searching.

We could also change over the bitmap to a char or u8 based array and lose 
all the sizeof(long) indexing headaches, resulting type casts, for 
anything but the actual bitmap_set/clear() calls, etc.?

I.e. now that most of the logic is byte granular, the basic data 
structure might as well reflect that?

> +	/*
> +	 * Update the per thread storage and the TSS bitmap. This must be
> +	 * done with preemption disabled to prevent racing against a
> +	 * context switch.
> +	 */
> +	preempt_disable();
> +	tss = this_cpu_ptr(&cpu_tss_rw);
>  
> +	if (!t->io_bitmap_ptr) {
> +		unsigned int tss_start = tss->io_zerobits_start;
> +		/*
> +		 * If the task did not use the I/O bitmap yet then the
> +		 * perhaps stale content in the TSS needs to be taken into
> +		 * account. If tss start is out of bounds the TSS storage
> +		 * does not contain a zero bit and it's sufficient just to
> +		 * copy the new range over.
> +		 */

s/tss/TSS

> +		if (tss_start < IO_BITMAP_BYTES) {
> +			unsigned int tss_end =  tss->io_zerobits_end;
> +			unsigned int copy_end = copy_start + copy_len;
> +
> +			copy_start = min(tss_start, copy_start);
> +			copy_len = max(tss_end, copy_end) - copy_start;
> +		}
> +	}
> +
> +	/* Copy the changed range over to the TSS bitmap */
> +	dst = (char *)tss->io_bitmap;
> +	src = (char *)bitmap;
> +	memcpy(dst + copy_start, src + copy_start, copy_len);
> +
> +	if (first >= IO_BITMAP_BITS) {
> +		/*
> +		 * If the resulting bitmap has all permissions dropped, clear
> +		 * TIF_IO_BITMAP and set the IO bitmap offset in the TSS to
> +		 * invalid. Deallocate both the new and the thread's bitmap.
> +		 */
> +		clear_thread_flag(TIF_IO_BITMAP);
> +		tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_INVALID;
> +		tofree = bitmap;
> +		bitmap = NULL;

BTW., wouldn't it be simpler to just say that if a thread uses IO ops 
even once, it gets a bitmap and that's it? I.e. we could further simplify 
this seldom used piece of code.

> +	} else {
>  		/*
> +		 * I/O bitmap contains zero bits. Set TIF_IO_BITMAP, make
> +		 * the bitmap offset valid and make sure that the TSS limit
> +		 * is correct. It might have been wreckaged by a VMEXiT.
>  		 */
> +		set_thread_flag(TIF_IO_BITMAP);
> +		tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_VALID;
>  		refresh_tss_limit();
>  	}

I'm wondering, shouldn't we call refresh_tss_limit() in both branches, or 
is a VMEXIT-wreckaged TSS limit harmless if we otherwise have 
io_bitmap_base set to IO_BITMAP_OFFSET_INVALID?


>  	/*
> +	 * Update the range in the thread and the TSS
>  	 *
> +	 * Get the byte position of the first zero bit and calculate
> +	 * the length of the range in which zero bits exist.
>  	 */
> +	start = first / 8;
> +	end = first < IO_BITMAP_BITS ? round_up(last, 8) / 8 : 0;
> +	t->io_zerobits_start = tss->io_zerobits_start = start;
> +	t->io_zerobits_end = tss->io_zerobits_end = end;
>  
>  	/*
> +	 * Finally exchange the bitmap pointer in the thread.
>  	 */
> +	bitmap = xchg(&t->io_bitmap_ptr, bitmap);
> +	preempt_enable();
>  
> +	kfree(bitmap);
> +	kfree(tofree);
>  
>  	return 0;


Thanks,

	Ingo