Re: [Qemu-devel] [RFC PATCH 1/1] linux-user: Probe the guest base for shared objects when needed

[Meador Inge]

On 06/12/2012 09:08 AM, Richard Henderson wrote:

> I think this is one of those cases where the -B or -R options
> (or QEMU_GUEST_BASE and QEMU_RESERVED_VA env variables) are the best
> way forward for whatever cpu you're emulating.  That or a change to
> the target's default ld script, not to link real executables quite so 
> low in the address space.

Per Richard's recommendation I experimented with -R for my use cases.  It seems
to mostly work, but for ARM GNU/Linux there is an issue that makes it awkward
to work with.

In particular, this commit [1] added validation for the guest base as a way to
ensure that the kernel-provided user mode helper functions on ARM can be mapped.
The validation function is invoked by 'probe_guest_base', but also in
main.c:3456 whenever -R or -B is used:

    if (reserved_va || have_guest_base) {
        if (!guest_validate_base(guest_base)) {
            fprintf(stderr, "Guest base/Reserved VA rejected by guest code\n");
            exit(1);
        }
    }

Thus we might be able to allocate the reserved VA region, but it might fail the
validation and exit.  I had this actually happen on many test cases when testing
'-R 128M' with portions of the GCC testsuite.

To solve this issue I experimented with performing a similar probing in 'main'
as in 'probe_guest_base' so that we can find a reserved VA region that also
passes validation.  If a region isn't found that can be validated, then QEMU
gives up.  Does this approach seem reasonable?


[1]
http://git.qemu.org/?p=qemu.git;a=commit;h=97cc75606aef406e90a243cdb25347039003e7f0

-- 
Meador Inge
CodeSourcery / Mentor Embedded
http://www.mentor.com/embedded-software