[Qemu-devel] [PATCH] target-arm: Implement a minimal set of cp14 debug registers

[Qemu-devel] [PATCH] target-arm: Implement a minimal set of cp14 debug registers

[Peter Maydell]

Newer ARM kernels try to probe for whether the CPU has hardware breakpoint
support. For this to work QEMU has to implement a minimal set of the cp14
debug registers. The architecture requires v7 cores to implement debug
and so there is no defined way to report its absence; however in practice
returning a zero DBGDIDR (ie with a reserved value for "debug architecture
version") should cause well-written hw debug users to do the right thing.
We also implement DBGDRAR and DBGDSAR as RAZ, indicating no memory mapped
debug components.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target-arm/translate.c |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+), 0 deletions(-)

diff --git a/target-arm/translate.c b/target-arm/translate.c
index dbd958b..d05859d 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -5769,6 +5769,34 @@ static int disas_cp14_read(CPUState * env, DisasContext *s, uint32_t insn)
     int rt = (insn >> 12) & 0xf;
     TCGv tmp;
 
+    /* Minimal set of debug registers, since we don't support debug */
+    if (op1 == 0 && crn == 0 && op2 == 0) {
+        switch (crm) {
+        case 0:
+            /* DBGDIDR: just RAZ. In particular this means the
+             * "debug architecture version" bits will read as
+             * a reserved value, which should cause Linux to
+             * not try to use the debug hardware.
+             */
+            tmp = tcg_const_i32(0);
+            store_reg(s, rt, tmp);
+            return 0;
+        case 1:
+        case 2:
+            /* DBGDRAR and DBGDSAR: v7 only. Always RAZ since we
+             * don't implement memory mapped debug components
+             */
+            if (ENABLE_ARCH_7) {
+                tmp = tcg_const_i32(0);
+                store_reg(s, rt, tmp);
+                return 0;
+            }
+            break;
+        default:
+            break;
+        }
+    }
+
     if (arm_feature(env, ARM_FEATURE_THUMB2EE)) {
         if (op1 == 6 && crn == 0 && crm == 0 && op2 == 0) {
             /* TEECR */
-- 
1.7.1

Re: [Qemu-devel] [PATCH] target-arm: Implement a minimal set of cp14 debug registers

[Peter Maydell]

On 22 February 2011 18:19, Peter Maydell <peter.maydell@linaro.org> wrote:
> +            tmp = tcg_const_i32(0);
> +            store_reg(s, rt, tmp);

This generates spurious "resource leak" warnings, because
store_reg() calls dead_tmp() so you can't store anything
you didn't get from new_tmp().

I think I'll do what Aurelien suggested and put together
a patch that implements the resource-leak debug as a proper
facility at the tcg level.

-- PMM

Re: [Qemu-devel] [PATCH] target-arm: Implement a minimal set of cp14 debug registers

[Aurelien Jarno]

On Tue, Feb 22, 2011 at 06:19:43PM +0000, Peter Maydell wrote:
> Newer ARM kernels try to probe for whether the CPU has hardware breakpoint
> support. For this to work QEMU has to implement a minimal set of the cp14
> debug registers. The architecture requires v7 cores to implement debug
> and so there is no defined way to report its absence; however in practice
> returning a zero DBGDIDR (ie with a reserved value for "debug architecture
> version") should cause well-written hw debug users to do the right thing.
> We also implement DBGDRAR and DBGDSAR as RAZ, indicating no memory mapped
> debug components.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  target-arm/translate.c |   28 ++++++++++++++++++++++++++++
>  1 files changed, 28 insertions(+), 0 deletions(-)

Thanks, applied.

> diff --git a/target-arm/translate.c b/target-arm/translate.c
> index dbd958b..d05859d 100644
> --- a/target-arm/translate.c
> +++ b/target-arm/translate.c
> @@ -5769,6 +5769,34 @@ static int disas_cp14_read(CPUState * env, DisasContext *s, uint32_t insn)
>      int rt = (insn >> 12) & 0xf;
>      TCGv tmp;
>  
> +    /* Minimal set of debug registers, since we don't support debug */
> +    if (op1 == 0 && crn == 0 && op2 == 0) {
> +        switch (crm) {
> +        case 0:
> +            /* DBGDIDR: just RAZ. In particular this means the
> +             * "debug architecture version" bits will read as
> +             * a reserved value, which should cause Linux to
> +             * not try to use the debug hardware.
> +             */
> +            tmp = tcg_const_i32(0);
> +            store_reg(s, rt, tmp);
> +            return 0;
> +        case 1:
> +        case 2:
> +            /* DBGDRAR and DBGDSAR: v7 only. Always RAZ since we
> +             * don't implement memory mapped debug components
> +             */
> +            if (ENABLE_ARCH_7) {
> +                tmp = tcg_const_i32(0);
> +                store_reg(s, rt, tmp);
> +                return 0;
> +            }
> +            break;
> +        default:
> +            break;
> +        }
> +    }
> +
>      if (arm_feature(env, ARM_FEATURE_THUMB2EE)) {
>          if (op1 == 6 && crn == 0 && crm == 0 && op2 == 0) {
>              /* TEECR */
> -- 
> 1.7.1
> 
> 
> 

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net