Re: [Qemu-devel] [PATCH] os: don't corrupt pre-existing memory-backend data with prealloc

[Eric Blake]

On 02/24/2017 11:27 AM, Daniel P. Berrange wrote:
> When using a memory-backend object with prealloc turned on, QEMU
> will memset() the first byte in every memory page to zero. While
> this might have been acceptable for memory backends associated
> with RAM, this corrupts application data for NVDIMMs.
> 
> Instead of setting every page to zero, read the current byte
> value and then just write that same value back, so we are not
> corrupting the original data. Directly write the value instead
> of memset()ing it, since there's no benefit to memset for a
> single byte write.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
> 

>          /* MAP_POPULATE silently ignores failures */
>          for (i = 0; i < numpages; i++) {
> -            memset(area + (hpagesize * i), 0, 1);
> +            /*
> +             * Read & write back the same value, so we don't
> +             * corrupt existinng user/app data that might be

s/existinng/existing/

> +             * stored.
> +             *
> +             * 'volatile' to stop compiler optimizing this away
> +             * to a no-op
> +             *
> +             * TODO: get a better solution from kernel so we
> +             * don't need to write at all so we don't cause
> +             * wear on the storage backing the region...
> +             */
> +            volatile char val = *(area + (hpagesize * i));
> +            *(area + (hpagesize * i)) = val;
>          }
>      }
>  
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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