Re: [PATCH 3/3] riscv: add trap and emulation for RDCYCLE

Ben Dooks
2024-09-18T09:07:45Z

On 18/09/2024 07:45, Andrew Jones wrote:
> On Tue, Sep 17, 2024 at 02:08:53PM GMT, Ben Dooks wrote:
>> Add a trap for RDCYCLE and emulate it as RDTIME instruciton.
>>
>> This is an initial PoC and should probably be made more generic
>> way of trapping and dealing with bad instructions
>>
>> Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
>> ---
>> arch/riscv/kernel/traps.c | 31 +++++++++++++++++++++++++++++++
>> 1 file changed, 31 insertions(+)
>>
>> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>> index 1c3fab272fd1..51ea28ebf54d 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -167,6 +167,35 @@ DO_ERROR_INFO(do_trap_insn_misaligned,
>> DO_ERROR_INFO(do_trap_insn_fault,
>> SIGSEGV, SEGV_ACCERR, "instruction access fault");
>>
>> +#define is_system(__i) (((__i) & 0x7f) == RVG_OPCODE_SYSTEM)
>
> We have riscv_insn_is_system()

Ah, didn't notice that.

>> +
>> +static bool riscv_try_csr_fixup_user(struct pt_regs *regs, u32 insn)
>> +{
>> + /* expecting a 4 byte CSR instruction (*/
>> + if (unlikely(GET_INSN_LENGTH(insn) != 4))
>> + return false;
>> +
>> + if (is_system(insn)) {
>> + u32 csr = insn >> RVG_SYSTEM_CSR_OFF;
>> + u32 rd = (insn >> RVG_RD_OPOFF) & RVG_RD_MASK;
>> + u32 rs = (insn >> RVG_RS1_OPOFF) & RVG_RS1_MASK;
>> + u32 funct3 = (insn >> RV_INSN_FUNCT3_OPOFF) & 0x7;
>
> There are are a few other macros in asm/insn.h that can be applied, such
> as RV_EXTRACT_RD_REG(), and more could be added if necessary.

will have a look at that.

>> +
>> + if (rs == 0 && funct3 == 2 && csr == CSR_CYCLE) {
>
> We could probably create a riscv_insn_is_csr_read() and RV_EXTRACT_CSR()
> for this.

Not sure if this is totally necessary. I will check for the relevant
read/write type from the CSR acces instructions

>> + u64 val = csr_read(CSR_TIME);
>> + /* we've got a RDCCLYE, emulated it with CSR_TIME */
>> +
>> + printk_ratelimited("PID %d: process using RDCYCLE, emulating with RDTIME\n", current->pid);
>
> If we add current->comm it may be easier to find applications that should
> be converted to CSR_TIME.

ok, will add.

>> +
>> + regs_set_register(regs, rd*sizeof(unsigned long), val);
>
> nit: spaces around the '*'

oops. will fix

>> + regs->epc += 4;
>> + return true;
>> + }
>> + }
>> +
>> + return false;
>> +}
>> +
>> asmlinkage __visible __trap_section void do_trap_insn_illegal(struct pt_regs *regs)
>> {
>> bool handled;
>> @@ -186,6 +215,8 @@ asmlinkage __visible __trap_section void do_trap_insn_illegal(struct pt_regs *re
>> }
>>
>> handled = riscv_v_first_use_handler(regs, insn);
>> + if (!handle)
> ^handled?
>
>> + handled = riscv_try_csr_fixup_user(regs, insn);
>>
>> local_irq_disable();
>>
>> --
>> 2.37.2.352.g3c44437643
>>
>>
>
> Thanks,
> drew

Thanks.

--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius