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

[Michal Privoznik]

On 02/23/2017 11:59 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.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
> 
> I'm unclear if this is actually still safe in practice ? Is the
> compiler permitted to optimize away the read+write since it doesn't
> change the memory value. I'd hope not, but I've been surprised
> before...
> 
> IMHO this is another factor in favour of requesting an API from
> the kernel to provide the prealloc behaviour we want.
> 
>  util/oslib-posix.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 35012b9..8f5b656 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -355,7 +355,8 @@ void os_mem_prealloc(int fd, char *area, size_t memory, 
> Error **errp)
>  
>          /* MAP_POPULATE silently ignores failures */
>          for (i = 0; i < numpages; i++) {
> -            memset(area + (hpagesize * i), 0, 1);
> +            char val = *(area + (hpagesize * i));
> +            memset(area + (hpagesize * i), 0, val);

I think you wanted:

memset(area + (hpagesize * i), val, 1);

because what you are suggesting will overwrite even more than the first
byte with zeroes.

Michal