Re: [PATCH v2 2/2] xentrace: handle sparse cpu ids correctly in xen trace buffer handling

[George Dunlap <george.dunlap@citrix.com>]

On 10/04/2018 11:51 AM, Juergen Gross wrote:
> The per-cpu buffers for Xentrace are addressed by cpu-id, but the info
> array for the buffers is sized only by number of online cpus. This
> might lead to crashes when using Xentrace with smt=0.
> 
> The t_info structure has to be sized based on nr_cpu_ids.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  tools/xentrace/xentrace.c | 2 +-
>  xen/common/trace.c        | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/xentrace/xentrace.c b/tools/xentrace/xentrace.c
> index 590a91e091..12497a16b4 100644
> --- a/tools/xentrace/xentrace.c
> +++ b/tools/xentrace/xentrace.c
> @@ -596,7 +596,7 @@ static unsigned int get_num_cpus(void)
>          exit(EXIT_FAILURE);
>      }
>  
> -    return physinfo.nr_cpus;
> +    return physinfo.max_cpu_id + 1;
>  }
>  
>  /**
> diff --git a/xen/common/trace.c b/xen/common/trace.c
> index 8cdc17b731..c079454c6a 100644
> --- a/xen/common/trace.c
> +++ b/xen/common/trace.c
> @@ -113,7 +113,7 @@ static int calculate_tbuf_size(unsigned int pages, uint16_t t_info_first_offset)
>      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 max_cpus = nr_cpu_ids;
>      unsigned int t_info_words;
>  
>      /* force maximum value for an unsigned type */
> @@ -151,11 +151,11 @@ static int calculate_tbuf_size(unsigned int pages, uint16_t t_info_first_offset)
>       * NB this calculation is correct, because t_info_first_offset is
>       * in words, not bytes, not bytes
>       */

This sounds a bit like song lyrics.

But that's not your fault, not your fault:

Reviewed-by: George Dunlap <george.dunlap@citrix.com>

I'll fix the comment on check-in.

 -George

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