Re: [PATCH v4 10/21] perf arm_spe: Fixup top byte for data virtual address

["André Przywara" <andre.przywara@arm.com>]

On 27/10/2020 03:09, Leo Yan wrote:
> To establish a valid address from the address packet payload and finally
> the address value can be used for parsing data symbol in DSO, current
> code uses 0xff to replace the tag in the top byte of data virtual
> address.
> 
> So far the code only fixups top byte for the memory layouts with 4KB
> pages, it misses to support memory layouts with 64KB pages.
> 
> This patch adds the conditions for checking bits [55:48] are 0xf0 or
> 0xfd, if detects the patterns it will fill 0xff into the top byte of the
> address, also adds comment to explain the fixing up.
> 
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
>  .../util/arm-spe-decoder/arm-spe-decoder.c    | 24 ++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-decoder.c b/tools/perf/util/arm-spe-decoder/arm-spe-decoder.c
> index 776b3e6628bb..e135ac01d94a 100644
> --- a/tools/perf/util/arm-spe-decoder/arm-spe-decoder.c
> +++ b/tools/perf/util/arm-spe-decoder/arm-spe-decoder.c
> @@ -24,7 +24,7 @@
>  
>  static u64 arm_spe_calc_ip(int index, u64 payload)
>  {
> -	u64 ns, el;
> +	u64 ns, el, val;
>  
>  	/* Instruction virtual address or Branch target address */
>  	if (index == SPE_ADDR_PKT_HDR_INDEX_INS ||
> @@ -45,8 +45,26 @@ static u64 arm_spe_calc_ip(int index, u64 payload)
>  		/* Clean tags */
>  		payload = SPE_ADDR_PKT_ADDR_GET_BYTES_0_6(payload);
>  
> -		/* Fill highest byte if bits [48..55] is 0xff */
> -		if (SPE_ADDR_PKT_ADDR_GET_BYTE_6(payload) == 0xffULL)
> +		/*
> +		 * Armv8 ARM (ARM DDI 0487F.c), chapter "D10.2.1 Address packet"
> +		 * defines the data virtual address payload format, the top byte
> +		 * (bits [63:56]) is assigned as top-byte tag; so we only can
> +		 * retrieve address value from bits [55:0].
> +		 *
> +		 * According to Documentation/arm64/memory.rst, if detects the
> +		 * specific pattern in bits [55:48] of payload which falls in
> +		 * the kernel space, should fixup the top byte and this allows
> +		 * perf tool to parse DSO symbol for data address correctly.
> +		 *
> +		 * For this reason, if detects the bits [55:48] is one of
> +		 * following values, will fill 0xff into the top byte:
> +		 *
> +		 *   - 0xff (for most kernel memory regions);
> +		 *   - 0xf0 (for kernel logical memory map with 64KB pages);
> +		 *   - 0xfd (for kasan shadow region with 64KB pages).
> +		 */
> +		val = SPE_ADDR_PKT_ADDR_GET_BYTE_6(payload);
> +		if (val == 0xffULL || val == 0xf0ULL || val == 0xfdULL)

But those values are just the beginning of the region used by the
kernel, aren't they? So the kernel logical map goes from 0xfff000.. to
0xfff7fff..., for instance.

But actually I wonder why were are so selective here? Wouldn't it just
suffice to look at bits [55:52] to be either 0 or F?

Cheers,
Andre

>  			payload |= 0xffULL << SPE_ADDR_PKT_ADDR_BYTE7_SHIFT;
>  
>  	/* Data access physical address */
>