Re: [Qemu-devel] [PATCH] linux-user: fix mips 32-on-64 prealloc case

[Alexander Graf]

On 03.01.2013, at 18:19, Peter Maydell wrote:

> On 3 January 2013 13:17, Alexander Graf <address@hidden> wrote:
>> MIPS only supports 31 bits of virtual address space for user space, so let's
>> make sure we stay within that limit with our preallocated memory block.
>> 
>> This fixes the MIPS user space targets when executed without command line
>> option.
> 
> This looks weird -- why should the guest care that we've reserved a
> 4GB block which it only uses half of? Or is the problem that host
> mmap() ends up handing out addresses from anywhere in the 4GB
> reserved area?

Even worse, it starts from the top IIRC.

MIPS uses the upper virtual address bit for kernel/user space indication. I'm 
not sure where exactly this logic falls apart in our case, but user space 
virtual addresses above 2GB are simple illegal in that world, so I wouldn't 
expect QEMU or a guest process to cope with them.


Alex