Re: [PATCH v4] gdb/arm: Handle lazy FPU state preservation

[Luis Machado <luis.machado@arm.com>]

Hi,

I think the current mechanism is better than the observer scheme.

On 10/2/22 15:51, Torbjörn SVENSSON wrote:
> Read LSPEN, ASPEN and LSPACT bits from FPCCR and use them together
> with FPCAR to identify if lazy FPU state preservation is active for
> the current frame.  See "Lazy context save of FP state", in B1.5.7,
> also ARM AN298, supported by Cortex-M4F architecture for details on
> lazy FPU register stacking.  The same conditions are valid for other
> Cortex-M cores with FPU.
> 
> This patch has been verified on a STM32F4-Discovery board by:
> a) writing a non-zero value (lets use 0x1122334455667788 as an
>     example) to all the D-registers in the main function
> b) configured the SysTick to fire
> c) in the SysTick_Handler, write some other value (lets use
>     0x0022446688aaccee as an example) to one of the D-registers (D0 as
>     an example) and then do "SVC #0"
> d) in the SVC_Handler, write some other value (lets use
>     0x0099aabbccddeeff) to one of the D-registers (D0 as an example)
> 
> In GDB, suspend the execution in the SVC_Handler function and compare
> the value of the D-registers for the SVC_handler frame and the
> SysTick_Handler frame.  With the patch, the value of the modified
> D-register (D0) should be the new value (0x009..eff) on the
> SVC_Handler frame, and the intermediate value (0x002..cee) for the
> SysTick_Handler frame.  Now compare the D-register value for the
> SysTick_Handler frame and the main frame.  The main frame should
> have the initial value (0x112..788).
> 
> 
> Example GDB session:
> 
> C:\>c:\dev\gdb\arm-none-eabi-gdb.exe C:\Users\foo\STM32Cube\Repository\STM32Cube_FW_F4_V1.27.0\Projects\STM32F4-Discovery\Examples\GPIO\GPIO_EXTI\SW4STM32\STM32F4-Discovery\Debug\STM32F4-Discovery.elf
> GNU gdb (GDB) 13.0.50.20221001-git
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> Type "show configuration" for configuration details.
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> Find the GDB manual and other documentation resources online at:
>      <http://www.gnu.org/software/gdb/documentation/>.
> 
> For help, type "help".
> Type "apropos word" to search for commands related to "word"...
> Reading symbols from C:\Users\foo\STM32Cube\Repository\STM32Cube_FW_F4_V1.27.0\Projects\STM32F4-Discovery\Examples\GPIO\GPIO_EXTI\SW4STM32\STM32F4-Discovery\Debug\STM32F4-Discovery.elf...
> (gdb) set pagination off
> (gdb) set confirm off
> (gdb) define lazy_dump
> Type commands for definition of "lazy_dump".
> End with a line saying just "end".
>>   shell echo
>>   shell echo "Content at $arg0"
>>   set $i = 0
>>   while $i <= $arg1
>   >    f $i
>   >    p/x $d0
>   >    p/x $d1
>   >    shell echo
>   >    set $i = $i + 1
>   >  end
>>   shell echo
>> end
> (gdb)
> (gdb) target remote :61234
> Remote debugging using :61234
> Reset_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/SW4STM32\startup_stm32f407xx.s:63
> 63        ldr   sp, =_estack     /* set stack pointer */
> (gdb) b main
> Breakpoint 1 at 0x80015cc: file C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/main.c, line 63.
> Note: automatically using hardware breakpoints for read-only addresses.
> (gdb) b SysTick_Handler
> Breakpoint 2 at 0x8001854: file C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c, line 150.
> (gdb) b SVC_Handler
> Breakpoint 3 at 0x8001808: file C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c, line 112.
> (gdb) c
> Continuing.
> 
> Breakpoint 1, main () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/main.c:63
> 63        HAL_Init();
> (gdb) lazy_dump main 0
> 
> Content at main
> #0  main () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/main.c:63
> 63        HAL_Init();
> $1 = 0x0
> $2 = 0x0
> 
> 
> (gdb) c
> Continuing.
> 
> Breakpoint 2, SysTick_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:150
> 150       HAL_IncTick();
> (gdb) lazy_dump "SysTick_Handler before vmov" 2
> 
> Content at "SysTick_Handler before vmov"
> #0  SysTick_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:150
> 150       HAL_IncTick();
> $3 = 0x1122334455667788
> $4 = 0x1122334455667788
> 
> #1  <signal handler called>
> $5 = 0x1122334455667788
> $6 = 0x1122334455667788
> 
> #2  main () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/main.c:120
> 120       while (1)
> $7 = 0x1122334455667788
> $8 = 0x1122334455667788
> 
> 
> (gdb) c
> Continuing.
> 
> Breakpoint 3, SVC_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:112
> 112               asm("push {r0, r1}");
> (gdb) lazy_dump "SVC_Handler before vmov" 4
> 
> Content at "SVC_Handler before vmov"
> #0  SVC_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:112
> 112               asm("push {r0, r1}");
> $9 = 0x22446688aaccee
> $10 = 0x22446688aaccee
> 
> #1  <signal handler called>
> $11 = 0x22446688aaccee
> $12 = 0x22446688aaccee
> 
> #2  SysTick_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:162
> 162     }
> $13 = 0x22446688aaccee
> $14 = 0x22446688aaccee
> 
> #3  <signal handler called>
> $15 = 0x22446688aaccee
> $16 = 0x22446688aaccee
> 
> #4  main () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/main.c:120
> 120       while (1)
> $17 = 0x1122334455667788
> $18 = 0x1122334455667788
> 
> 
> (gdb) s 8
> 122       HAL_GPIO_EXTI_Callback(KEY_BUTTON_PIN);
> (gdb) lazy_dump "SVC_Handler after vmov" 4
> 
> Content at "SVC_Handler after vmov"
> #0  SVC_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:122
> 122       HAL_GPIO_EXTI_Callback(KEY_BUTTON_PIN);
> $19 = 0x99aabbccddeeff
> $20 = 0x99aabbccddeeff
> 
> #1  <signal handler called>
> $21 = 0x99aabbccddeeff
> $22 = 0x99aabbccddeeff
> 
> #2  SysTick_Handler () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/stm32f4xx_it.c:162
> 162     }
> $23 = 0x22446688aaccee
> $24 = 0x22446688aaccee
> 
> #3  <signal handler called>
> $25 = 0x22446688aaccee
> $26 = 0x22446688aaccee
> 
> #4  main () at C:/Users/foo/STM32Cube/Repository/STM32Cube_FW_F4_V1.27.0/Projects/STM32F4-Discovery/Examples/GPIO/GPIO_EXTI/Src/main.c:120
> 120       while (1)
> $27 = 0x1122334455667788
> $28 = 0x1122334455667788
> 
> 
> 
> 
> 
> Signed-off-by: Torbjörn SVENSSON  <torbjorn.svensson@foss.st.com>
> Signed-off-by: Yvan ROUX  <yvan.roux@foss.st.com>
> ---
>   gdb/arch/arm.h |  7 ++++++-
>   gdb/arm-tdep.c | 51 ++++++++++++++++++++++++++++++++++----------------
>   2 files changed, 41 insertions(+), 17 deletions(-)
> 
> diff --git a/gdb/arch/arm.h b/gdb/arch/arm.h
> index 36757493406..d384b952144 100644
> --- a/gdb/arch/arm.h
> +++ b/gdb/arch/arm.h
> @@ -115,7 +115,12 @@ enum system_register_address : CORE_ADDR
>     /* M-profile Floating-Point Context Control Register address, defined in
>        ARMv7-M (Section B3.2.2) and ARMv8-M (Section D1.2.99) reference
>        manuals.  */
> -  FPCCR = 0xe000ef34
> +  FPCCR = 0xe000ef34,
> +
> +  /* M-profile Floating-Point Context Address Register address, defined in
> +     ARMv7-M (Section B3.2.2) and ARMv8-M (Section D1.2.98) reference
> +     manuals.  */
> +  FPCAR = 0xe000ef38
>   };
>   
>   /* Instruction condition field values.  */
> diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
> index 2810232fcb8..b08ee096f97 100644
> --- a/gdb/arm-tdep.c
> +++ b/gdb/arm-tdep.c
> @@ -3588,27 +3588,43 @@ arm_m_exception_cache (struct frame_info *this_frame)
>         if (extended_frame_used)
>   	{
>   	  ULONGEST fpccr;
> +	  ULONGEST fpcar;
>   
>   	  /* Read FPCCR register.  */
>   	  gdb_assert (safe_read_memory_unsigned_integer (FPCCR,
>   							 ARM_INT_REGISTER_SIZE,
>   							 byte_order, &fpccr));
> -	  bool fpccr_ts = bit (fpccr, 26);
>   
> -	  /* This code does not take into account the lazy stacking, see "Lazy
> -	     context save of FP state", in B1.5.7, also ARM AN298, supported
> -	     by Cortex-M4F architecture.
> -	     To fully handle this the FPCCR register (Floating-point Context
> -	     Control Register) needs to be read out and the bits ASPEN and
> -	     LSPEN could be checked to setup correct lazy stacked FP registers.
> -	     This register is located at address 0xE000EF34.  */
> +	  /* Read FPCAR register.  */
> +	  gdb_assert (safe_read_memory_unsigned_integer (FPCAR,
> +							 ARM_INT_REGISTER_SIZE,
> +							 byte_order, &fpcar));

I'm not sure about this assertion. Can we handle this gracefully without crashing GDB?

> +	  bool fpccr_aspen = bit (fpccr, 31);
> +	  bool fpccr_lspen = bit (fpccr, 30);
> +	  bool fpccr_ts = bit (fpccr, 26);
> +	  bool fpccr_lspact = bit (fpccr, 0);
> +
> +	  /* The LSPEN and ASPEN bits indicate if the lazy state preservation
> +	     for FP registers is enabled or disabled.  The LSPACT bit indicate,
> +	     together with FPCAR, if the lazy state preservation feature is
> +	     active for the current frame or for another frame.
> +	     See "Lazy context save of FP state", in B1.5.7, also ARM AN298,
> +	     supported by Cortex-M4F architecture for details.  */
> +	  bool fpcar_points_to_this_frame = ((unwound_sp + sp_r0_offset + 0x20)

I'd define "addr = unwound_sp + sp_r0_offset + 0x20" and use it here...

> +					     == (fpcar & ~0x7));
> +	  bool read_fp_regs_from_stack = (!(fpccr_aspen && fpccr_lspen
> +					    && fpccr_lspact
> +					    && fpcar_points_to_this_frame));
>   
>   	  /* Extended stack frame type used.  */
> -	  CORE_ADDR addr = unwound_sp + sp_r0_offset + 0x20;
> -	  for (int i = 0; i < 8; i++)
> +	  if (read_fp_regs_from_stack)
>   	    {
> -	      cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
> -	      addr += 8;
> +	      CORE_ADDR addr = unwound_sp + sp_r0_offset + 0x20;

... and here.

> +	      for (int i = 0; i < 8; i++)
> +		{
> +		  cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
> +		  addr += 8;
> +		}
>   	    }
>   	  cache->saved_regs[ARM_FPSCR_REGNUM].set_addr (unwound_sp
>   							+ sp_r0_offset + 0x60);
> @@ -3617,11 +3633,14 @@ arm_m_exception_cache (struct frame_info *this_frame)
>   	      && fpccr_ts)
>   	    {
>   	      /* Handle floating-point callee saved registers.  */
> -	      addr = unwound_sp + sp_r0_offset + 0x68;
> -	      for (int i = 8; i < 16; i++)
> +	      if (read_fp_regs_from_stack)
>   		{
> -		  cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
> -		  addr += 8;
> +		  CORE_ADDR addr = unwound_sp + sp_r0_offset + 0x68;

Then adjust addr here if we are using a different address.

> +		  for (int i = 8; i < 16; i++)
> +		    {
> +		      cache->saved_regs[ARM_D0_REGNUM + i].set_addr (addr);
> +		      addr += 8;
> +		    }
>   		}
>   
>   	      arm_cache_set_active_sp_value (cache, tdep,

Otherwise LGTM.

Thanks for putting this together.