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

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Message-ID: <ba2432e4-cfa9-62a8-626e-97aab44c9ffe@arm.com>
Date:   Thu, 21 Sep 2023 13:07:29 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>
Cc:     amit.kachhap@....com, angelogioacchino.delregno@...labora.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux@...linux.org.uk,
        matthias.bgg@...il.com, yj.chiang@...iatek.com
Subject: Re: [PATCH] ARM: vfp: Add vudot opcode to VFP undef hook

On 21/09/2023 3:13 am, Mark-PK Tsai wrote:
>> 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? ;)
> 
> The hwcap for dotprod has been exported since commit:
> 
> 62ea0d873af3 ARM: 9269/1: vfp: Add hwcap for FEAT_DotProd
> 
>>
>> 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
> 
> Were the mentioned series related to this commit?
> 
> 62ea0d873af3 ARM: 9269/1: vfp: Add hwcap for FEAT_DotProd

Oh, that did get merged? My apologies, I grepped for the hwcaps in 
arch/arm but somehow failed to spot that some definitions did exist, so 
assumed it hadn't been; not sure what went wrong there :(

In that case, we definitely want this tagged as a fix, and to make sure 
we double-check for any equivalent fixes still needed for the other 
features too. Sorry again for the confusion.

>> 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.
> 
> Agree.
> 
>>
>>> 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.
> 
> I thought that maybe someone in arm need this.
> But it seems a bit noisy so I will remove the link from v2.

Yeah, even within Arm most of us don't have permission to access the 
support system.

Cheers,
Robin.

>>
>>> 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?
> 
> You're right.
> I will set both cpsr_mask and cpsr_val to 0 and use single entry,
> as you suggested, in the v2 patch.
> 
> Thanks.
> 
>>
>> Thanks,
>> Robin.
>>
>>>    }};
>>>    
>>>    static struct undef_hook vfp_support_hook = {