Re: [Qemu-devel] Re: [PATCH] Fix segfault with ram_size > 4095M without kvm

[Ryan Harper <ryanh@us.ibm.com>]

* Aurelien Jarno <aurelien@aurel32.net> [2010-12-25 16:37]:
> On Wed, Dec 08, 2010 at 04:27:45PM -0200, Luiz Capitulino wrote:
> > On Wed, 08 Dec 2010 12:23:12 -0600
> > Anthony Liguori <aliguori@linux.vnet.ibm.com> wrote:
> > 
> > > On 12/08/2010 12:01 PM, Luiz Capitulino wrote:
> > > > Currently, x86_64-softmmu qemu segfaults when trying to use>  4095M memsize.
> > > > This patch adds a simple check and error message (much like the 2047 limit on
> > > > 32-bit hosts) on ram_size in the control path after we determine we're
> > > > not using kvm
> > > >
> > > > Upstream qemu-kvm is affected if using the -no-kvm option; this patch address
> > > > the segfault there as well.
> > > >
> > > > Signed-off-by: Ryan Harper<ryanh@us.ibm.com>
> > > > Signed-off-by: Aurelien Jarno<aurelien@aurel32.net>
> > > > ---
> > > > NOTE: this patch was applied in the v0.12.x branch, but it seems it got
> > > >        lost for master
> > > >    
> > > 
> > > No, it was intentional.  We should fix the segv, this is not a known 
> > > limitation but rather a bug.
> > 
> > A TCG bug, I presume?
> > 
> 
> Do you have more details about this issue and how to reproduce it? 

At the time of the bug, it was something simple like:

qemu-system-x86_64 -m 4097 -hda /dev/null

we'd get an imediate segfault.  As you say, I'm not seeing it now on
current git; I'll see about bisecting to see if we did get a fix for the
issue.

> 
> Support for more than 4GB of memory has been added a few years ago, 
> and I am not able to reproduce the problem anymore (I have booted a 
> 64-bit guest with 6GB of RAM, and make sure the guest use the whole 
> memory). I guess TCG itself is fine, but there might be a bug in
> the MMU emulation in some cases.
> 
> I also noticed that now i386-softmmu has been artificially limited to
> 2047MB. Tthis configuration used to support up to 64GB of RAM (PAE)
> on 64-bit hosts.
> 
> -- 
> Aurelien Jarno                          GPG: 1024D/F1BCDB73
> aurelien@aurel32.net                 http://www.aurel33.net

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com