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

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

* Aurelien Jarno <aurelien@aurel32.net> [2010-03-04 15:27]:
> 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:

Thanks for looking into this.

> 
> 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.

Anthony, do you want me to resend and rebase against 0.12-stable?


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