[Qemu-devel] cpu_memory_rw_debug doesn't work on MIPS?

[Qemu-devel] cpu_memory_rw_debug doesn't work on MIPS?

[Igor R]

If I understand correctly, the most advanced MMU that QEMU emulates
for MIPS is "R4000-style" MMU - i.e. a "software-managed" TLB, where
on TLB miss QEMU just emulates exception that should be handled by the
guest OS. So, QEMU doesn't walk through the page directory, like it
does when emulating e.g. x86.
While this approach works for the guest code, it results in inability
to read guest virtual memory "externally" - from within a monitor
command, for debugging purposes etc. That is, cpu_memory_rw_debug()
doesn't work reliably for mapped segments - it fails because of TLB
miss, but no one tries to fill the TLB.
If all the above correct, is there any work-around that can be used to
read the guest memory in qemu-system-mips?

Thanks.

Re: [Qemu-devel] cpu_memory_rw_debug doesn't work on MIPS?

[Leon Alrae]

On 25/02/16 18:56, Igor R wrote:
> If I understand correctly, the most advanced MMU that QEMU emulates
> for MIPS is "R4000-style" MMU - i.e. a "software-managed" TLB, where
> on TLB miss QEMU just emulates exception that should be handled by the
> guest OS. So, QEMU doesn't walk through the page directory, like it
> does when emulating e.g. x86.

Yes, most of QEMU MIPS CPUs support R4000-style MMU. MIPS Hardware Table
Walker was introduced in release 5 of MIPS Architecture, and implemented
in P5600. However, HTW isn't present in QEMU yet.

> While this approach works for the guest code, it results in inability
> to read guest virtual memory "externally" - from within a monitor
> command, for debugging purposes etc. That is, cpu_memory_rw_debug()
> doesn't work reliably for mapped segments - it fails because of TLB
> miss, but no one tries to fill the TLB.

Indeed, given virtual address (belonging to mapped segments like kseg2)
without mapping is quite useless without involving guest's TLB-miss handler.

> If all the above correct, is there any work-around that can be used to
> read the guest memory in qemu-system-mips?

QEMU monitor provides also physical memory dump but obviously you need
to know the physical address. Otherwise - if you want to use virtual
address from mapped segment - you need to read the memory while there's
valid TLB entry.

Leon

> 
> Thanks.
>