Re: [Xen-devel] [PATCH 1 of 4] xentrace: reduce trace buffer size to something mfn_offset can reach

[George Dunlap <dunlapg@xxxxxxxxx>]

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

Sorry for the delay!
 -George

On Tue, May 10, 2011 at 3:32 PM, Olaf Hering <olaf@xxxxxxxxx> wrote:
> # HG changeset patch
> # User Olaf Hering <olaf@xxxxxxxxx>
> # Date 1305037380 -7200
> # Node ID 8ac937fa527b28243227193bf4749feb3a234c2c
> # Parent  19452acd23045f40c4e18437f0a60f016757e5bd
> xentrace: reduce trace buffer size to something mfn_offset can reach
>
> The start of the array which holds the list of mfns for each cpus
> tracebuffer is stored in an unsigned short. This limits the total amount
> of pages for each cpu as the number of active cpus increases.
>
> Update the math in calculate_tbuf_size() to apply also this rule to the
> max number of trace pages. Without this change the index can overflow.
>
> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
>
> diff -r 19452acd2304 -r 8ac937fa527b xen/common/trace.c
> --- a/xen/common/trace.c        Fri May 06 11:15:35 2011 +0100
> +++ b/xen/common/trace.c        Tue May 10 16:23:00 2011 +0200
> @@ -112,11 +112,14 @@ static int calculate_tbuf_size(unsigned
>     typeof(dummy_size.prod) max_size;
>     struct t_info dummy_pages;
>     typeof(dummy_pages.tbuf_size) max_pages;
> +    typeof(dummy_pages.mfn_offset[0]) max_mfn_offset;
> +    unsigned int max_cpus = num_online_cpus();
>     unsigned int t_info_words;
>
>     /* force maximum value for an unsigned type */
>     max_size = -1;
>     max_pages = -1;
> +    max_mfn_offset = -1;
>
>     /* max size holds up to n pages */
>     max_size /= PAGE_SIZE;
> @@ -124,6 +127,18 @@ static int calculate_tbuf_size(unsigned
>     if ( max_size < max_pages )
>         max_pages = max_size;
>
> +    /*
> +     * max mfn_offset holds up to n pages per cpu
> +     * The array of mfns for the highest cpu can start at the maximum value
> +     * mfn_offset can hold. So reduce the number of cpus and also the 
> mfn_offset.
> +     */
> +    max_mfn_offset -= t_info_first_offset - 1;
> +    max_cpus--;
> +    if ( max_cpus )
> +        max_mfn_offset /= max_cpus;
> +    if ( max_mfn_offset < max_pages )
> +        max_pages = max_mfn_offset;
> +
>     if ( pages > max_pages )
>     {
>         printk(XENLOG_INFO "xentrace: requested number of %u pages "
>
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>

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