On 06/27/2012 07:47 PM, Paul Brook wrote: >> On 28.06.2012, at 02:06, Paul Brook wrote: >>>>> openSUSE uses a version patched so that IIUC 3G are reserved. >>>>> Just today this failed on a system where swap got disabled and the >>>>> mmap() thus failed. >>>> >>>> Err... why? We map with MAP_NORESERVE, so swap shouldn't matter... >>> >>> I can't say if it's the same cause, but we fail with "ulimit -v 4046848". >>> >>> Incidentally, it seems a strange that we only reserve 0xf7000000 bytes, >>> not the full 4G. >> >> Uh, I think that was because of the vdso shared page that is allocated on >> top of -R. > > That can't be right. The whole point of -R is that it defines all the guest > accessible virtual address space. The surrounding space is liable to be used > by something else, and we must not make any assumptions about it. > > Further inspection shows that guest_validate_base contains some extremely > bogus code. > > If the guest needs something at the top of its address space then we need to > offset address zero within the block, and ensure accesses wrap appropriately.
'guest_validate_base' is currently called for three reasons: (1) in main.c when using -B, (2) in main.c when using -R after mapping the reserved va region, and (3) and when probing for a guest base in probe_guest_base. For case (1) I suppose things are pretty much the same -- we just need to map the extra region when needed (e.g. for the ARM kernel helpers). For case (2) maybe we can do a probing similar to what I mentioned here [1], but taking into account what you stated above and ensuring that the probing finds a single region for the request va region size and any needed extra stuff. Case (3) is mostly the same as (2) but we are probing for a guest base with a region size deduced from looking at the image we are loading. I suppose it is still OK to map two regions here. The single region only applies to -R? Thoughts? [1] http://lists.nongnu.org/archive/html/qemu-devel/2012-06/msg04589.html -- Meador Inge CodeSourcery / Mentor Embedded http://www.mentor.com/embedded-software
