Re: [win-pv-devel] [PATCH] Ensure Hash mapping has at least 1 entry when Toeplitz is selected.

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: win-pv-devel [mailto:win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf Of owen.smith@xxxxxxxxxx
> Sent: 14 October 2016 15:25
> To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Owen Smith <owen.smith@xxxxxxxxxx>
> Subject: [win-pv-devel] [PATCH] Ensure Hash mapping has at least 1 entry
> when Toeplitz is selected.
> 
> From: Owen Smith <owen.smith@xxxxxxxxxx>
> 
> Its possible to select Toeplitz hashing without specifiying the hash
> map. A divide-by-zero BSOD can occur when a packet is sent before
> the hash map is created. Also add an ASSERT to detect potential
> divide-by-zeros in checked builds.
> 
> Signed-off-by: Owen Smith <owen.smith@xxxxxxxxxx>
> ---
>  src/xenvif/frontend.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/src/xenvif/frontend.c b/src/xenvif/frontend.c
> index f6570e8..0b51fd1 100644
> --- a/src/xenvif/frontend.c
> +++ b/src/xenvif/frontend.c
> @@ -1863,6 +1863,12 @@ __FrontendUpdateHash(
>          break;
> 
>      case XENVIF_PACKET_HASH_ALGORITHM_TOEPLITZ:
> +        if (Hash->Size == 0) {
> +            // if Algorithm is set without setting a hash map,
> +            // we need to ensure the hash map has 1 entry (0)
> +            Hash->Size = 1;
> +            Hash->Mapping[0] = 0;
> +        }

I think we can avoid this...

>          Size = Hash->Size;
>          Mapping = Hash->Mapping;
>          Flags = Hash->Flags;
> @@ -2151,6 +2157,7 @@ FrontendGetQueue(
>          break;
> 
>      case XENVIF_PACKET_HASH_ALGORITHM_TOEPLITZ:
> +        ASSERT3U(Frontend->Hash.Size, != , 0);

...if we simply add a check on Hash.Size here (and select queue 0 if the size 
is 0), instead of the assert.

  Paul

>          Queue = Frontend->Hash.Mapping[Value % Frontend->Hash.Size];
>          break;
> 
> --
> 2.8.3
> 
> 
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