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

Aurelien Jarno
2010-03-04T21:27:28Z

On Tue, Feb 23, 2010 at 06:02:15PM +0100, Aurelien Jarno wrote:
> 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.

I have looked into that, and actually one patch to get full support for
> 4GB of memory was not merged:

diff --git a/exec.c b/exec.c
index 8389c54..b0bb058 100644
--- a/exec.c
+++ b/exec.c
@@ -166,7 +166,7 @@ typedef struct PhysPageDesc {
*/
#define L1_BITS (TARGET_VIRT_ADDR_SPACE_BITS - L2_BITS - TARGET_PAGE_BITS)
#else
-#define L1_BITS (32 - L2_BITS - TARGET_PAGE_BITS)
+#define L1_BITS (TARGET_PHYS_ADDR_SPACE_BITS - L2_BITS - TARGET_PAGE_BITS)
#endif

#define L1_SIZE (1 << L1_BITS)

While this patch is acceptable for qemu i386, it creates a big L1 table
for x86_64 or other 64-bit architectures, resulting in huge memory
overhead.

The recent multilevel tables patches from Richard Henderson should fix
the problem for HEAD (I haven't found time to look at them in details).

As this is not something we really want to backport, your patch makes
sense in stable-0.12.

> > 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
>
>
>

--
Aurelien Jarno GPG: 1024D/F1BCDB73
aurelien@aurel32.net http://www.aurel32.net