Re: [PATCH v4 2/2] arm/monitor: Add support for 'info tlb' command

[Peter Maydell]

On Wed, 18 Aug 2021 at 13:00, Bin Meng <bmeng.cn@gmail.com> wrote:
>
> On Wed, Aug 18, 2021 at 7:48 PM NDNF <arkaisp2021@gmail.com> wrote:
> >
> > This adds hmp 'info tlb' command support for the arm platform.
> > The limitation is that this only implements a page walker for
> > ARMv8-A AArch64 Long Descriptor format, 32bit addressing is
> > not supported yet.
> >
> > Signed-off-by: Changbin Du <changbin.du@gmail.com>
> > Signed-off-by: Ivanov Arkady <arkaisp2021@gmail.com>
> > ---
> >  hmp-commands-info.hx |    3 +
> >  target/arm/monitor.c |  170 
> > ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 172 insertions(+), 1 deletion(-)
> >
>
> I believe this is not TLB on ARM, but PTE?
>
> RISC-V implemented a "info mem" for page table walk.

We call our "give page table walk" monitor command "info tlb"
for some reason (probably historical). From the docs:
https://qemu-project.gitlab.io/qemu/system/monitor.html
  info tlb
      Show virtual to physical memory mappings
  info mem
      Show the active virtual memory mappings.

Looking at x86's output, "info tlb" gives lines like this:

ffffffffc04f6000: 00000000b2576000 -G-DA----
ffffffffc04f7000: 00000000b2579000 -G-DA----
ffffffffc04f8000: 00000000bac74000 XG-DA----
ffffffffc04f9000: 00000000b258b000 XG-DA----
ffffffffc04fa000: 00000000b248b000 XG-DA---W
ffffffffc04fb000: 00000000b2431000 XG-DA---W
ffffffffc0502000: 00000000bb69c000 -G-DA----
ffffffffc0503000: 00000000b27d3000 XG-DA----

which is a dump of the page table, with one line per page,
giving the vaddr, the physaddr and associated flag information.

"info mem" gives lines like this:

ffffffffc04f6000-ffffffffc04fa000 0000000000004000 -r-
ffffffffc04fa000-ffffffffc04fc000 0000000000002000 -rw
ffffffffc0502000-ffffffffc0504000 0000000000002000 -r-

which just give "this range of virtual addresses of this
length have these permissions". You can see that it
coalesces multiple adjacent pages with the same permissions
into a single line.

The only architectures which implement "info mem"
are i386 and riscv. If riscv has given the command
different semantics to i386 that would be unfortunate.

"info tlb" is implemented by i386, m68k, nios2, ppc, sh4,
sparc, xtensa.

It's not clear to me that "info mem" is all that useful -- you
can figure out the same info from "info tlb".

-- PMM