linux-kernel - Re: [PATCH] ARM: vfp: Add vudot opcode to VFP undef hook

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Date:   Wed, 20 Sep 2023 19:49:19 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>,
        Russell King <linux@...linux.org.uk>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Cc:     yj.chiang@...iatek.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        Amit Daniel Kachhap <amit.kachhap@....com>
Subject: Re: [PATCH] ARM: vfp: Add vudot opcode to VFP undef hook

On 2023-09-20 09:39, Mark-PK Tsai wrote:
> Add vudot opcode to the VFP undef hook to fix the
> potentially undefined instruction error when the
> user space executes vudot instruction.

Did the kernel expose a hwcap to say that the dot product extension is 
supported? I'm pretty sure it didn't, so why would userspace expect this 
to work? ;)

IIRC Amit was looking at defining the hwcaps to align with arm64 compat, 
but I believe that series faltered since most of them weren't actually 
needed (and I think at that point it was still missing the VFP support 
code parts). It would be nice if someone could pick up and combine both 
efforts and get this done properly; fill in *all* the hwcaps and 
relevant handling for extensions which Cortex-A55 supports (since 
there's definitely more than just VUDOT), and then hopefully we're done 
for good.

> Before this commit, kernel didn't handle the undef exception
> caused by vudot and didn't enable VFP in lazy VFP context
> switch code like other NEON instructions.
> This led to the occurrence of the undefined instruction
> error as following:
> 
> [  250.741238 ] 0904 (26902): undefined instruction: pc=004014ec
> ...
> [  250.741287 ] PC is at 0x4014ec
> [  250.741298 ] LR is at 0xb677874f
> [  250.741303 ] pc : [<004014ec>]    lr : [<b677874f>]    psr: 80070010
> [  250.741309 ] sp : beffedb0  ip : b67d7864  fp : beffee58
> [  250.741314 ] r10: 00000000  r9 : 00000000  r8 : 00000000
> [  250.741319 ] r7 : 00000001  r6 : 00000001  r5 : beffee90  r4 : 00401470
> [  250.741324 ] r3 : beffee20  r2 : beffee30  r1 : beffee40  r0 : 004003a8
> [  250.741331 ] Flags: Nzcv  IRQs on  FIQs on  Mode USER_32  ISA ARM Segment user
> [  250.741339 ] Control: 10c5383d  Table: 32d0406a  DAC: 00000055
> [  250.741348 ] Code: f4434aef f4610aef f4622aef f4634aef (fc620df4)
> 
> Below is the assembly of the user program:
> 
> 0x4014dc <+108>: vst1.64 {d20, d21}, [r3:128]
> 0x4014e0 <+112>: vld1.64 {d16, d17}, [r1:128]
> 0x4014e4 <+116>: vld1.64 {d18, d19}, [r2:128]
> 0x4014e8 <+120>: vld1.64 {d20, d21}, [r3:128] --> switch out
> 0x4014ec <+124>: vudot.u8 q8, q9, q10         <-- switch in, and FPEXC.EN = 0
>                                                    SIGILL(illegal instruction)
> 
> Link: https://services.arm.com/support/s/case/5004L00000XsOjP

Linking to your private support case is not useful to upstream. Even I 
can't open that link.

> Signed-off-by: Mark-PK Tsai <mark-pk.tsai@...iatek.com>
> ---
>   arch/arm/vfp/vfpmodule.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
> index 7e8773a2d99d..7eab8d1019d2 100644
> --- a/arch/arm/vfp/vfpmodule.c
> +++ b/arch/arm/vfp/vfpmodule.c
> @@ -788,6 +788,12 @@ static struct undef_hook neon_support_hook[] = {{
>   	.cpsr_mask	= PSR_T_BIT,
>   	.cpsr_val	= 0,
>   	.fn		= vfp_support_entry,
> +}, {
> +	.instr_mask	= 0xffb00000,
> +	.instr_val	= 0xfc200000,
> +	.cpsr_mask	= PSR_T_BIT,
> +	.cpsr_val	= 0,
> +	.fn		= vfp_support_entry,
>   }, {
>   	.instr_mask	= 0xef000000,
>   	.instr_val	= 0xef000000,
> @@ -800,6 +806,12 @@ static struct undef_hook neon_support_hook[] = {{
>   	.cpsr_mask	= PSR_T_BIT,
>   	.cpsr_val	= PSR_T_BIT,
>   	.fn		= vfp_support_entry,
> +}, {
> +	.instr_mask	= 0xffb00000,
> +	.instr_val	= 0xfc200000,
> +	.cpsr_mask	= PSR_T_BIT,
> +	.cpsr_val	= PSR_T_BIT,
> +	.fn		= vfp_support_entry,

Why have two entries conditional on each possible value of one bit for 
otherwise identical encodings? Surely it suffices to set both cpsr_mask 
and cpsr_val to 0?

Thanks,
Robin.

>   }};
>   
>   static struct undef_hook vfp_support_hook = {