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

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

* Aurelien Jarno <aurelien@aurel32.net> [2010-02-23 11:37]:
> Ryan Harper a écrit :
> > 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.
> 
> It looks like workarounding the real bug. At some point both
> i386-softmmu (via PAE) and x86_64-softmmu were able to support > 4GB of
> memory. I remember adding the support long time ago, and testing it with
> 32GB of emulated RAM.

Indeed it was a workaround.  I thought it was reasonable since we cap
the size for 32-bit at 2047; but Anthony mentioned that on 64-bit hosts
some targets had >4G support so we should fix the segfault.  here is the
backtrace I got from the core file:

Core was generated by `./x86_64-softmmu/qemu-system-x86_64 -m 4096'.
Program terminated with signal 11, Segmentation fault.
#0  0x00000000004d1a59 in tb_alloc_page (tb=0x7f33d111d010, n=0, page_addr=4295094272)
    at /home/rharper/work/git/qemu/exec.c:1125
1125        tb->page_next[n] = p->first_tb;
(gdb) bt
#0  0x00000000004d1a59 in tb_alloc_page (tb=0x7f33d111d010, n=0, page_addr=4295094272)
    at /home/rharper/work/git/qemu/exec.c:1125
#1  0x00000000004d1bf1 in tb_link_phys (tb=0x7f33d111d010, phys_pc=4295098352, phys_page2=18446744073709551615)
    at /home/rharper/work/git/qemu/exec.c:1215
#2  0x00000000004d1612 in tb_gen_code (env=0x2180ed0, pc=4294967280, cs_base=4294901760, flags=68, cflags=0)
    at /home/rharper/work/git/qemu/exec.c:913
#3  0x00000000004d849c in tb_find_slow (pc=4294967280, cs_base=4294901760, flags=68)
    at /home/rharper/work/git/qemu/cpu-exec.c:161
#4  0x00000000004d85b2 in tb_find_fast () at /home/rharper/work/git/qemu/cpu-exec.c:182
#5  0x00000000004d8cdc in cpu_x86_exec (env1=0x2180ed0) at /home/rharper/work/git/qemu/cpu-exec.c:579
#6  0x000000000040d686 in qemu_cpu_exec (env=0x2180ed0) at /home/rharper/work/git/qemu/vl.c:3895
#7  0x000000000040d76b in tcg_cpu_exec () at /home/rharper/work/git/qemu/vl.c:3924
#8  0x000000000040da39 in main_loop () at /home/rharper/work/git/qemu/vl.c:4042
#9  0x0000000000411a2f in main (argc=3, argv=0x7fff5782ab08, envp=0x7fff5782ab28)
    at /home/rharper/work/git/qemu/vl.c:6102


> 
> 
> > Signed-off-by: Ryan Harper <ryanh@us.ibm.com>
> > ---
> >  vl.c |    6 ++++++
> >  1 files changed, 6 insertions(+), 0 deletions(-)
> > 
> > diff --git a/vl.c b/vl.c
> > index db7a178..a659e98 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -5760,6 +5760,12 @@ int main(int argc, char **argv, char **envp)
> >              fprintf(stderr, "failed to initialize KVM\n");
> >              exit(1);
> >          }
> > +    } else {
> > +        /* without kvm enabled, we can only support 4095 MB RAM */
> > +        if (ram_size > (4095UL << 20)) {
> > +            fprintf(stderr, "qemu: without kvm support at most 4095 MB RAM can be simulated\n");
> > +            exit(1);
> > +        }
> >      }
> >  
> >      if (qemu_init_main_loop()) {
> 
> 
> -- 
> Aurelien Jarno                          GPG: 1024D/F1BCDB73
> aurelien@aurel32.net                 http://www.aurel32.net
> --
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-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com