Re: [Qemu-devel] Regression caused by d2f39ad "exec.c: Ensure right alignment also for file backed ram"

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 21 Oct 2016 16:28:01 +0800
Haozhong Zhang <haozhong.zhang@intel.com> wrote:

> Hi,
> 
> Commit d2f39ad "exec.c: Ensure right alignment also for file backed
> ram" added an additional alignment requirement besides to the previous
> page size for the size of the backend file. On x86, the alignment
> requirement for the size of the backend file is changed from 4KB in
> QEMU 2.6 to 2MB in QEMU 2.7.
> 
> This change immediately breaks the usage of "-object memory-backend-file,...,size=$SIZE"
> on x86, where $SIZE is multiple of 4KB but not 2MB. It works on QEMU 2.6.
> Could this be considered as a regression?
> 
> The commit message shows it's for s390. I'm wondering whether the same
> regression happens on s390 and ARM. If not, could I fix the regression
> on x86 by keeping using the original alignment only on x86, e.g.
I don't see a nice way to work around this regression, so perhaps
we should tree 2.7 as broken (wrt migration when *dimm devices with file
backends with pagesize < 2Mb are used) and unbreak 2.8 and onwards.

Could you send a proper patch to list with commit message
explaining what is broken and how + CLI example.

> 
> modified   exec.c
> @@ -1254,7 +1254,11 @@ static void *file_ram_alloc(RAMBlock *block,
>      }
>  
>      block->page_size = qemu_fd_getpagesize(fd);
> +#if defined(__x86_64__) || defined(__i386__)
> +    block->mr->align = block->page_size;
> +#else
>      block->mr->align = MAX(block->page_size, QEMU_VMALLOC_ALIGN);
maybe it's worth to do other way around i.e. for special s390 case
do above and for the rest leave page_size alignment?


> +#endif
>  
>      if (memory < block->page_size) {
>          error_setg(errp, "memory size 0x" RAM_ADDR_FMT " must be equal to "
> 
> 
> Thanks,
> Haozhong
>