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Date: Mon, 1 Mar 2010 20:32:39 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [Qemu-devel] [patch uq/master 2/2] Add option to use file backed
	guest memory
Message-ID: <20100301233239.GA21535@amt.cnet>
References: <20100224211117.958583246@amt.cnet>
	<20100224211507.913712224@amt.cnet>
	<201002280128.16649.paul@codesourcery.com>
	<20100301232508.GA13703@amt.cnet>
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To: Paul Brook <paul@codesourcery.com>
Cc: john cooper <john.cooper@redhat.com>, qemu-devel@nongnu.org, kvm@vger.kernel.org, avi@redhat.com

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
> > >+@item -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.