arm: singlestep bug

arm: singlestep bug

[Andrew Jones]

Hello TCG developers,

We have new debug test cases in kvm-unit-tests thanks to Ricardo Koller.
The singlestep test cases are failing with TCG. Enabling TCG debug outputs
the error

  TCG hflags mismatch (current:(0x000004a1,0x0000000000004000) rebuilt:(0x000004a3,0x0000000000004000)

I noticed that the test passed on an older QEMU, so I bisected it and
found commit e979972a6a17 ("target/arm: Rely on hflags correct in
cpu_get_tb_cpu_state"), which unfortunately doesn't tell us anything
that the above error message didn't say already (apparently we can't
currently depend on hflags being correct wrt singlestep at this point).

Thanks,
drew

Re: arm: singlestep bug

[Alex Bennée]

Andrew Jones <drjones@redhat.com> writes:

> Hello TCG developers,
>
> We have new debug test cases in kvm-unit-tests thanks to Ricardo
> Koller.

Yay tests ;-)

> The singlestep test cases are failing with TCG. Enabling TCG debug outputs
> the error
>
>   TCG hflags mismatch (current:(0x000004a1,0x0000000000004000) rebuilt:(0x000004a3,0x0000000000004000)

This shows that:

  FIELD(TBFLAG_ANY, SS_ACTIVE, 1, 1)

should be set but wasn't cached.

> I noticed that the test passed on an older QEMU, so I bisected it and
> found commit e979972a6a17 ("target/arm: Rely on hflags correct in
> cpu_get_tb_cpu_state"), which unfortunately doesn't tell us anything
> that the above error message didn't say already (apparently we can't
> currently depend on hflags being correct wrt singlestep at this
> point).

Fortunately this is intended - the enable-debug always recalculates the
hflags (and expensive operation) and makes it pretty easy to spot where
we failed to call arm_rebuild_hflags(). You can do this with the normal
debug tools or my new favourite tool (for short programs) using the
execlog plugin.

  0, 0x40080a24, 0x52840020, "movz w0, #0x2001"
  0, 0x40080a28, 0x2a010000, "orr w0, w0, w1"
  0, 0x40080a2c, 0xd5100240, "msr mdscr_el1, x0"
  0, 0x40080a30, 0xd5033fdf, "isb "
  0, 0x40080a34, 0x350001f4, "cbnz w20, #0x40080a70"
  TCG hflags mismatch (current:(0x000004a1,0x0000000000004000) rebuilt:(0x000004a3,0x0000000000004000)

This is a touch weird though because any msr write does trigger a
rebuild of the flags. See handle_sys():

    if (!isread && !(ri->type & ARM_CP_SUPPRESS_TB_END)) {
        /*
         * A write to any coprocessor regiser that ends a TB
         * must rebuild the hflags for the next TB.
         */
        TCGv_i32 tcg_el = tcg_const_i32(s->current_el);
        gen_helper_rebuild_hflags_a64(cpu_env, tcg_el);
        tcg_temp_free_i32(tcg_el);
        /*
         * We default to ending the TB on a coprocessor register write,
         * but allow this to be suppressed by the register definition
         * (usually only necessary to work around guest bugs).
         */
        s->base.is_jmp = DISAS_UPDATE_EXIT;
    }

And indeed in rr I can see it working though the tail end of
helper_rebuild_flags_a64() but it seems arm_singlestep_active() returns
false at this point. This ultimately fails at
aa64_generate_debug_exceptions():

    /*
     * Same EL to same EL debug exceptions need MDSCR_KDE enabled
     * while not masking the (D)ebug bit in DAIF.
     */
    debug_el = arm_debug_target_el(env);

    if (cur_el == debug_el) {
        return extract32(env->cp15.mdscr_el1, 13, 1)
            && !(env->daif & PSTATE_D);
    }

And if I look at the objdump it is indeed the instruction we never
completed:

     a34:       350001f4        cbnz    w20, a70 <ss_start+0x34>
     a38:       d50348ff        msr     daifclr, #0x8

So if I force the flag generation on manipulating daif:

--8<---------------cut here---------------start------------->8---
modified   target/arm/helper-a64.c
@@ -83,12 +83,14 @@ void HELPER(msr_i_daifset)(CPUARMState *env, uint32_t imm)
 {
     daif_check(env, 0x1e, imm, GETPC());
     env->daif |= (imm << 6) & PSTATE_DAIF;
+    arm_rebuild_hflags(env);
 }
 
 void HELPER(msr_i_daifclear)(CPUARMState *env, uint32_t imm)
 {
     daif_check(env, 0x1f, imm, GETPC());
     env->daif &= ~((imm << 6) & PSTATE_DAIF);
+    arm_rebuild_hflags(env);
 }
 
--8<---------------cut here---------------end--------------->8---

  I now get a working test:

  env QEMU=$HOME/lsrc/qemu.git/builds/all.debug/qemu-system-aarch64 ./run_tests.sh -g debug
  PASS debug-bp (6 tests)
  PASS debug-bp-migration (7 tests)
  PASS debug-wp (8 tests)
  PASS debug-wp-migration (9 tests)
  PASS debug-sstep (1 tests)
  PASS debug-sstep-migration (1 tests)

(I was momentarily confused when debug-sstep failed, but that was I'd
forgotten to point to my build, the system 5.2 qemu is broken in this
regard).

I'll spin up a proper patch.

Side note:

  ad5fb8830150071487025b3594a7b1bf218d12d8 is the first bad commit
  commit ad5fb8830150071487025b3594a7b1bf218d12d8
  Author: Zixuan Wang <zixuanwang@google.com>
  Date:   Mon Oct 4 13:49:19 2021 -0700

breaks the running on kvm-unit-test for me, I needed to patch:

--8<---------------cut here---------------start------------->8---
modified   run_tests.sh
@@ -31,7 +31,8 @@ specify the appropriate qemu binary for ARCH-run.
 EOF
 }
 
-RUNTIME_arch_run="./$TEST_SUBDIR/run"
+RUNTIME_arch_run="./$TEST_DIR/run"
+#RUNTIME_arch_run="./$TEST_SUBDIR/run"
 source scripts/runtime.bash
 
 # require enhanced getopt
--8<---------------cut here---------------end--------------->8---


-- 
Alex Bennée

Re: arm: singlestep bug

[Andrew Jones]

On Wed, Feb 02, 2022 at 11:16:46AM +0000, Alex Bennée wrote:
...
> Side note:
> 
>   ad5fb8830150071487025b3594a7b1bf218d12d8 is the first bad commit
>   commit ad5fb8830150071487025b3594a7b1bf218d12d8
>   Author: Zixuan Wang <zixuanwang@google.com>
>   Date:   Mon Oct 4 13:49:19 2021 -0700
> 
> breaks the running on kvm-unit-test for me, I needed to patch:
> 
> --8<---------------cut here---------------start------------->8---
> modified   run_tests.sh
> @@ -31,7 +31,8 @@ specify the appropriate qemu binary for ARCH-run.
>  EOF
>  }
>  
> -RUNTIME_arch_run="./$TEST_SUBDIR/run"
> +RUNTIME_arch_run="./$TEST_DIR/run"
> +#RUNTIME_arch_run="./$TEST_SUBDIR/run"
>  source scripts/runtime.bash
>  
>  # require enhanced getopt
> --8<---------------cut here---------------end--------------->8---
>

You need to rerun ./configure to get a new config.mak file.

Thanks,
drew