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

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Message-ID: <Y44PmvL878v57CzZ@xhacker>
Date:   Mon, 5 Dec 2022 23:34:50 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Andrew Jones <ajones@...tanamicro.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Heiko Stuebner <heiko@...ech.de>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 01/13] riscv: fix jal offsets in patched alternatives

On Mon, Dec 05, 2022 at 03:57:10PM +0100, Andrew Jones wrote:
> On Mon, Dec 05, 2022 at 01:46:20AM +0800, Jisheng Zhang wrote:
> > 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@...nel.org>
> > ---
> >  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) | \
>                                                                  ^ RV_J_IMM_10_1_OPOFF

Good catch! I was lucky when testing the whole series ;) will fix it in
newer version.

> 
> > +	 ((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))
> 
> Should put () around value

good idea, previously, I just follow to_jalr_imm(), will update it in
newer version.

> 
> > +
> > +void riscv_alternative_fix_jal(void *alt_ptr, unsigned int len,
> > +			       int patch_offset)
> > +{
> > +	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);
> 
> It'd be nice if this had a define.
> 
> > +
> > +		/* 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);
> >  		}
> >  	}
> >  }
> > -- 
> > 2.37.2
> >
> 
> Thanks,
> drew