Re: [PATCH v2 01/13] riscv: fix jal offsets in patched alternatives

[Conor Dooley]

Heiko, Jisheng,

On Mon, Dec 05, 2022 at 11:40:44PM +0800, Jisheng Zhang wrote:
> On Mon, Dec 05, 2022 at 04:31:08PM +0100, Heiko Stübner wrote:
> > Am Sonntag, 4. Dezember 2022, 18:46:20 CET schrieb Jisheng Zhang:
> > > Alternatives live in a different section, so offsets used by jal
> > > instruction will point to wrong locations after the patch got applied.
> > > 
> > > Similar to arm64, adjust the location to consider that offset.
> > > 
> > > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > > ---
> > >  arch/riscv/include/asm/alternative.h |  2 ++
> > >  arch/riscv/kernel/alternative.c      | 38 ++++++++++++++++++++++++++++
> > >  arch/riscv/kernel/cpufeature.c       |  3 +++
> > >  3 files changed, 43 insertions(+)
> > > 
> > > diff --git a/arch/riscv/include/asm/alternative.h b/arch/riscv/include/asm/alternative.h
> > > index c58ec3cc4bc3..33eae9541684 100644
> > > --- a/arch/riscv/include/asm/alternative.h
> > > +++ b/arch/riscv/include/asm/alternative.h
> > > @@ -29,6 +29,8 @@ void apply_module_alternatives(void *start, size_t length);
> > >  
> > >  void riscv_alternative_fix_auipc_jalr(void *alt_ptr, unsigned int len,
> > >  				      int patch_offset);
> > > +void riscv_alternative_fix_jal(void *alt_ptr, unsigned int len,
> > > +			       int patch_offset);
> > >  
> > >  struct alt_entry {
> > >  	void *old_ptr;		 /* address of original instruciton or data  */
> > > diff --git a/arch/riscv/kernel/alternative.c b/arch/riscv/kernel/alternative.c
> > > index 292cc42dc3be..9d88375624b5 100644
> > > --- a/arch/riscv/kernel/alternative.c
> > > +++ b/arch/riscv/kernel/alternative.c
> > > @@ -125,6 +125,44 @@ void riscv_alternative_fix_auipc_jalr(void *alt_ptr, unsigned int len,
> > >  	}
> > >  }
> > >  
> > > +#define to_jal_imm(value)						\
> > > +	(((value & (RV_J_IMM_10_1_MASK << RV_J_IMM_10_1_OFF)) << RV_I_IMM_11_0_OPOFF) | \
> > > +	 ((value & (RV_J_IMM_11_MASK << RV_J_IMM_11_OFF)) << RV_J_IMM_11_OPOFF) | \
> > > +	 ((value & (RV_J_IMM_19_12_OPOFF << RV_J_IMM_19_12_OFF)) << RV_J_IMM_19_12_OPOFF) | \
> > > +	 ((value & (1 << RV_J_IMM_SIGN_OFF)) << RV_J_IMM_SIGN_OPOFF))
> > > +
> > > +void riscv_alternative_fix_jal(void *alt_ptr, unsigned int len,
> > > +			       int patch_offset)
> > > +{
> > 
> > I think we might want to unfiy this into a common function like
> > 
> > 	riscv_alternative_fix_offsets(...)
> > 
> > so that we only run through the code block once
> > 
> > 	for (i = 0; i < num_instr; i++) {
> > 		if (riscv_insn_is_auipc_jalr(inst1, inst2)) {
> > 			riscv_alternative_fix_auipc_jalr(...)
> > 			continue;
> > 		}
> > 
> > 		if (riscv_insn_is_jal(inst)) {
> > 			riscv_alternative_fix_jal(...)
> > 			continue;
> > 		}
> > 	}
> > 
> > This would also remove the need from calling multiple functions
> > after patching alternatives.
> 
> Yesterday, I also wanted to unify the two instruction fix into
> one. But that would need to roll back the
> riscv_alternative_fix_auipc_jalr() to your v1 version. And IMHO,
> it's better if you can split the Zbb string optimizations series
> into two: one for alternative improvements, another for Zbb. Then
> we may get the alternative improvements and this inst extension
> series merged in v6.2-rc1.

Heiko, perhaps you can correct me here:

Last Wednesday you & Palmer agreed that it was too late in the cycle to
apply any of the stuff touching alternatives?
If I do recall correctly, gives plenty of time to sort out any
interdependent changes here.

Could easily be misremembering, wouldn't be the first time!

Thanks,
Conor.

> > > +	int num_instr = len / sizeof(u32);
> > > +	unsigned int call;
> > > +	int i;
> > > +	int imm;
> > > +
> > > +	for (i = 0; i < num_instr; i++) {
> > > +		u32 inst = riscv_instruction_at(alt_ptr, i);
> > > +
> > > +		if (!riscv_insn_is_jal(inst))
> > > +			continue;
> > > +
> > > +		/* get and adjust new target address */
> > > +		imm = RV_EXTRACT_JTYPE_IMM(inst);
> > > +		imm -= patch_offset;
> > > +
> > > +		/* pick the original jal */
> > > +		call = inst;
> > > +
> > > +		/* drop the old IMMs, all jal imm bits sit at 31:12 */
> > > +		call &= ~GENMASK(31, 12);
> > > +
> > > +		/* add the adapted IMMs */
> > > +		call |= to_jal_imm(imm);
> > > +
> > > +		/* patch the call place again */
> > > +		patch_text_nosync(alt_ptr + i * sizeof(u32), &call, 4);
> > > +	}
> > > +}
> > > +
> > >  /*
> > >   * This is called very early in the boot process (directly after we run
> > >   * a feature detect on the boot CPU). No need to worry about other CPUs
> > > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > > index ba62a4ff5ccd..c743f0adc794 100644
> > > --- a/arch/riscv/kernel/cpufeature.c
> > > +++ b/arch/riscv/kernel/cpufeature.c
> > > @@ -324,6 +324,9 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> > >  			riscv_alternative_fix_auipc_jalr(alt->old_ptr,
> > >  							 alt->alt_len,
> > >  							 alt->old_ptr - alt->alt_ptr);
> > > +			riscv_alternative_fix_jal(alt->old_ptr,
> > > +						  alt->alt_len,
> > > +						  alt->old_ptr - alt->alt_ptr);
> > >  		}
> > >  	}
> > >  }
> > > 
> > 
> > 
> > 
> >

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