Re: [Qemu-devel] [patch uq/master 2/2] Add option to use file backed guest memory

[Marcelo Tosatti]

On Mon, Mar 01, 2010 at 08:25:08PM -0300, Marcelo Tosatti wrote:
> Hi Paul,
> 
> Thank you for reviewing.
> 
> On Sun, Feb 28, 2010 at 01:28:16AM +0000, Paul Brook wrote:
> > IMHO it would be better to check the mem_path != NULL here, rather that 
> > burying the check in file_ram_alloc.
> > 
> > >+    if (memory < hpagesize) {
> > >+        return NULL;
> > >+    }
> > 
> > Ah, so it's actually "allocate memory in $path, if you feel like it". Good 
> > job 
> > we aren't relying on this for correctness.  At minimum I recommend 
> > documenting 
> > this heuristic.
> 
> More like "allocate memory in $path, if it its larger than a hugepage."
> 
> Huge pages are an optimization.
> 
> > 
> > >+    if (!new_block->host) {
> > > #if defined(TARGET_S390X) && defined(CONFIG_KVM)
> > >-    /* XXX S390 KVM requires the topmost vma of the RAM to be < 256GB */
> > 
> > By my reading this implies -mempath is probably broken on s390 KVM?
> > 
> > >+DEF("mem-path", HAS_ARG, QEMU_OPTION_mempath,
> > >+    "-mem-path FILE  provide backing storage for guest RAM\n")
> > >+STEXI
> > >address@hidden -mem-path @var{path}
> > >+Allocate guest RAM from a temporarily created file in @var{path}.
> > >+ETEXI
> > 
> > You should mention that this is only useful when PATH happens to be a linux 
> > hugetlbfs mount.
> 
> It can be used with a file, since its mapped as MAP_PRIVATE.

I meant non hugetlbfs backed file.