RE: MIPS simulator is broken

[Matthew Fortune <Matthew.Fortune@imgtec.com>]

Mike Frysinger <vapier@gentoo.org> writes:
> since 64-bit address aren't actually being used in the 32-bit env, why
> bother using them ?  seems like it'd be much easier to just use 32-bit
> addresses and be done.

Hi Mike,

The problem here is fairly common and seems to boil down to a
misunderstanding at some level of the MIPS trick for 32-bit running on
64-bit architectures.

I agree that the address translation logic for MIPS seems weird but I
also don’t think it should not get changed just because it looks odd
without understanding why it is that way. As such for the time being I
propose reverting both changes to MIPS sim to get it working again:

    Revert "sim: mips: delete mmu stubs to move to common sim_{read,write}"

    This reverts commit 26f8bf63bf36f9062a5cc1afacf71462a4abe0c8.

    Revert "sim: mips: workaround 32-bit addr sign extensions"

    This reverts commit b36d953bced0a4fecdde1823abac70ed7038ee95.

I'd assume this is OK given it 'fixes' the regression despite taking the
code back to its unusual, but working, state.

I don't fully understand GNUSIM internals so please bear with me while
I get up to speed...

Let's assume we just delete the masking of address in address_translation:

diff --git a/sim/mips/sim-main.c b/sim/mips/sim-main.c
index 916769e..8cf5743 100644
--- a/sim/mips/sim-main.c
+++ b/sim/mips/sim-main.c
@@ -68,7 +68,7 @@ address_translation (SIM_DESC sd,

   /* For a simple (flat) memory model, we simply pass virtual
      addressess through (mostly) unchanged. */
-  vAddr &= 0xFFFFFFFF;
+//  vAddr &= 0xFFFFFFFF;

   *pAddr = vAddr;              /* default for isTARGET */
   *CCA = Uncached;             /* not used for isHOST */

Where we could aim for is that when simulating a 64-bit architecture
then all addresses (including those coming from o32 or n32 applications)
should be seen as 64-bit and sign extended (NOT zero extended) from the
32-bit values seen in the ELF.

This means code in an o32 ELF with address 0x80010000 should be loaded
at 0xffffffff80010000 and executed from that 64-bit address. When
presenting addresses to the user the upper 32-bits can be discarded as
they are irrelevant but internally in the sim they could be represented.

It seems this is how things work and I see sections being loaded at
sign extended 64-bit addresses addresses but even when I claim to have
a memory region at that 64-bit address I still get the read to unmapped
address error as the code does not appear to get loaded:

run --memory-region 0xffffffff80010000,0x10000  sanity.s.x
Loading section .text, size 0x60 lma 0xffffffff80010000
Loading section .MIPS.abiflags, size 0x18 lma 0x400098
Loading section .data, size 0x1a lma 0xffffffff80010060
mips-core: 4 byte read to unmapped address 0xffffffff80020000 at 0xffffffff80020000
program stopped with signal 10 (User defined signal 1).

The trace output shows this:

insn:     0x80010000 ---   _start         nop              - SLLb
insn:     0x80010004 ---   _start         nop              - SLLb
insn:     0x80010008 ---   _fail          nop              - SLLb
insn:     0x8001000c ---   _fail          nop              - SLLb
insn:     0x80010010 ---   _fail          nop              - SLLb
insn:     0x80010014 ---   _fail          nop              - SLLb

Can you help me understand why the code does not get loaded and/or
if there is somewhere else we may need to educate about sign extended
addresses?

Thanks,
Matthew