linux-kernel - Re: [PATCH v3] LoongArch: Remove vcsr in loongarch

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Date:   Thu, 07 Jul 2022 09:30:40 +0100
From:   "Jiaxun Yang" <jiaxun.yang@...goat.com>
To:     "Qi Hu" <huqi@...ngson.cn>, "Huacai Chen" <chenhuacai@...nel.org>
Cc:     "Xuerui Wang" <kernel@...0n.name>, loongarch@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] LoongArch: Remove vcsr in loongarch_fpu



在2022年7月7日七月 上午8:35，Qi Hu写道：
> On 2022/7/7 14:45, Huacai Chen wrote:
>> Hi, all,
>>
>> I have rewritten the commit message. If no problem, this patch will be
>> queued for loongarch-fixes.
>> --
>> The `vcsr` only exists in the old hardware design, it isn't used in any
>> shipped hardware from Loongson-3A5000 on. FPU and LSX/LASX both use the
>> `fcsr` as their control and status registers now. For example, the RM
>> control bit in fcsr0 is shared by FPU, LSX and LASX.
>>
>> Particularly, fcsr16 to fcsr31 are reserved for LSX/LASX now, access to
>> these registers has no visible effect if LSX/LASX is enabled, and will
>> cause SXD/ASXD exceptions if LSX/LASX is not enabled.
>>
>> So, mentions of vcsr are obsolete in the first place, let's remove them.
>
> Or simply say this?
>
> Remove VCSR from "loongarch_fpu" which is used for debugging.

Then you it will be a cross talk vulnerability if you simply ignore it in kernel.

Processes can create a covert channel for surreptitious data exchange based on VCSR like m1racles[1] for apple m1.

Thanks
[1]: https://m1racles.com/

>> --
>> Huacai
>>
>> On Wed, Jul 6, 2022 at 7:29 PM huqi <huqi@...ngson.cn> wrote:
>>> From: Qi Hu <huqi@...ngson.cn>
>>>
>>> The `vcsr` is not used anymore. Remove this member from `loongarch_fpu`.
>>>
>>>  From 3A5000(LoongArch), `vcsr` is removed in hardware. FP and LSX/LASX
>>> both use `fcsr` as their csr.
>>>
>>> Particularly, fcsr from $r16 to $r31 are reserved for LSX/LASX, an
>>> using the registers in this area will cause SXD/ASXD if LSX/LASX is
>>> not enabled.
>>>
>>> Signed-off-by: Qi Hu <huqi@...ngson.cn>
>>> ---
>>> V3:
>>> - Modify commit message to conform to the format.
>>> V2:
>>> - Add more details in the commit message.
>>> ---
>>>   arch/loongarch/include/asm/fpregdef.h  |  1 -
>>>   arch/loongarch/include/asm/processor.h |  2 --
>>>   arch/loongarch/kernel/asm-offsets.c    |  1 -
>>>   arch/loongarch/kernel/fpu.S            | 10 ----------
>>>   4 files changed, 14 deletions(-)
>>>
>>> diff --git a/arch/loongarch/include/asm/fpregdef.h b/arch/loongarch/include/asm/fpregdef.h
>>> index adb16e4b43b0..b6be527831dd 100644
>>> --- a/arch/loongarch/include/asm/fpregdef.h
>>> +++ b/arch/loongarch/include/asm/fpregdef.h
>>> @@ -48,6 +48,5 @@
>>>   #define fcsr1  $r1
>>>   #define fcsr2  $r2
>>>   #define fcsr3  $r3
>>> -#define vcsr16 $r16
>>>
>>>   #endif /* _ASM_FPREGDEF_H */
>>> diff --git a/arch/loongarch/include/asm/processor.h b/arch/loongarch/include/asm/processor.h
>>> index 1d63c934b289..57ec45aa078e 100644
>>> --- a/arch/loongarch/include/asm/processor.h
>>> +++ b/arch/loongarch/include/asm/processor.h
>>> @@ -80,7 +80,6 @@ BUILD_FPR_ACCESS(64)
>>>
>>>   struct loongarch_fpu {
>>>          unsigned int    fcsr;
>>> -       unsigned int    vcsr;
>>>          uint64_t        fcc;    /* 8x8 */
>>>          union fpureg    fpr[NUM_FPU_REGS];
>>>   };
>>> @@ -161,7 +160,6 @@ struct thread_struct {
>>>           */                                                     \
>>>          .fpu                    = {                             \
>>>                  .fcsr           = 0,                            \
>>> -               .vcsr           = 0,                            \
>>>                  .fcc            = 0,                            \
>>>                  .fpr            = {{{0,},},},                   \
>>>          },                                                      \
>>> diff --git a/arch/loongarch/kernel/asm-offsets.c b/arch/loongarch/kernel/asm-offsets.c
>>> index bfb65eb2844f..20cd9e16a95a 100644
>>> --- a/arch/loongarch/kernel/asm-offsets.c
>>> +++ b/arch/loongarch/kernel/asm-offsets.c
>>> @@ -166,7 +166,6 @@ void output_thread_fpu_defines(void)
>>>
>>>          OFFSET(THREAD_FCSR, loongarch_fpu, fcsr);
>>>          OFFSET(THREAD_FCC,  loongarch_fpu, fcc);
>>> -       OFFSET(THREAD_VCSR, loongarch_fpu, vcsr);
>>>          BLANK();
>>>   }
>>>
>>> diff --git a/arch/loongarch/kernel/fpu.S b/arch/loongarch/kernel/fpu.S
>>> index 75c6ce0682a2..a631a7137667 100644
>>> --- a/arch/loongarch/kernel/fpu.S
>>> +++ b/arch/loongarch/kernel/fpu.S
>>> @@ -146,16 +146,6 @@
>>>          movgr2fcsr      fcsr0, \tmp0
>>>          .endm
>>>
>>> -       .macro sc_save_vcsr base, tmp0
>>> -       movfcsr2gr      \tmp0, vcsr16
>>> -       EX      st.w \tmp0, \base, 0
>>> -       .endm
>>> -
>>> -       .macro sc_restore_vcsr base, tmp0
>>> -       EX      ld.w \tmp0, \base, 0
>>> -       movgr2fcsr      vcsr16, \tmp0
>>> -       .endm
>>> -
>>>   /*
>>>    * Save a thread's fp context.
>>>    */
>>> --
>>> 2.36.1
>>>
>>>

-- 
- Jiaxun