Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte
On Tue, Dec 01, 2020 at 09:48:36PM -0800, Alexei Starovoitov wrote: > On Tue, Dec 1, 2020 at 4:12 AM Brendan Jackman wrote: > > > > On Sat, Nov 28, 2020 at 05:14:05PM -0800, Alexei Starovoitov wrote: > > > On Fri, Nov 27, 2020 at 05:57:27PM +, Brendan Jackman wrote: > > > > The JIT case for encoding atomic ops is about to get more > > > > complicated. In order to make the review & resulting code easier, > > > > let's factor out some shared helpers. > > > > > > > > Signed-off-by: Brendan Jackman > > > > --- > > > > arch/x86/net/bpf_jit_comp.c | 39 ++--- > > > > 1 file changed, 23 insertions(+), 16 deletions(-) > > > > > > > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c > > > > index 94b17bd30e00..a839c1a54276 100644 > > > > --- a/arch/x86/net/bpf_jit_comp.c > > > > +++ b/arch/x86/net/bpf_jit_comp.c > > > > @@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, > > > > u32 r2, int off) > > > > *pprog = prog; > > > > } > > > > > > > > +/* > > > > + * Emit a REX byte if it will be necessary to address these registers > > > > > > What is "REX byte" ? > > > May be rename it to maybe_emit_mod() ? > > > > Er, this is the REX prefix as described in > > https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefix > > > > Would maybe_emit_mod be accurate? In my mind "mod" is a field in the > > ModR/M byte which comes _after_ the opcode. Before developing this > > patchset I knew almost nothing about x86, so maybe I'm missing something > > about the general terminology? > > I wrote the JIT without looking into the manual and without studying > the terminology. > Why? Because it was not necessary. I still don't see a reason why > that obscure terminology needs to be brought in into the code. > 'mod' to me is a 'modifier'. Nothing to do with intel's modrm thing. OK, calling it maybe_emit_mod(pprog, dst_reg, src_reg)
Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte
On Tue, Dec 1, 2020 at 4:12 AM Brendan Jackman wrote: > > On Sat, Nov 28, 2020 at 05:14:05PM -0800, Alexei Starovoitov wrote: > > On Fri, Nov 27, 2020 at 05:57:27PM +, Brendan Jackman wrote: > > > The JIT case for encoding atomic ops is about to get more > > > complicated. In order to make the review & resulting code easier, > > > let's factor out some shared helpers. > > > > > > Signed-off-by: Brendan Jackman > > > --- > > > arch/x86/net/bpf_jit_comp.c | 39 ++--- > > > 1 file changed, 23 insertions(+), 16 deletions(-) > > > > > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c > > > index 94b17bd30e00..a839c1a54276 100644 > > > --- a/arch/x86/net/bpf_jit_comp.c > > > +++ b/arch/x86/net/bpf_jit_comp.c > > > @@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, > > > u32 r2, int off) > > > *pprog = prog; > > > } > > > > > > +/* > > > + * Emit a REX byte if it will be necessary to address these registers > > > > What is "REX byte" ? > > May be rename it to maybe_emit_mod() ? > > Er, this is the REX prefix as described in > https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefix > > Would maybe_emit_mod be accurate? In my mind "mod" is a field in the > ModR/M byte which comes _after_ the opcode. Before developing this > patchset I knew almost nothing about x86, so maybe I'm missing something > about the general terminology? I wrote the JIT without looking into the manual and without studying the terminology. Why? Because it was not necessary. I still don't see a reason why that obscure terminology needs to be brought in into the code. 'mod' to me is a 'modifier'. Nothing to do with intel's modrm thing.
Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte
On Sat, Nov 28, 2020 at 05:14:05PM -0800, Alexei Starovoitov wrote:
> On Fri, Nov 27, 2020 at 05:57:27PM +, Brendan Jackman wrote:
> > The JIT case for encoding atomic ops is about to get more
> > complicated. In order to make the review & resulting code easier,
> > let's factor out some shared helpers.
> >
> > Signed-off-by: Brendan Jackman
> > ---
> > arch/x86/net/bpf_jit_comp.c | 39 ++---
> > 1 file changed, 23 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 94b17bd30e00..a839c1a54276 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, u32
> > r2, int off)
> > *pprog = prog;
> > }
> >
> > +/*
> > + * Emit a REX byte if it will be necessary to address these registers
>
> What is "REX byte" ?
> May be rename it to maybe_emit_mod() ?
Er, this is the REX prefix as described in
https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefix
Would maybe_emit_mod be accurate? In my mind "mod" is a field in the
ModR/M byte which comes _after_ the opcode. Before developing this
patchset I knew almost nothing about x86, so maybe I'm missing something
about the general terminology?
> > + */
> > +static void maybe_emit_rex(u8 **pprog, u32 reg_rm, u32 reg_reg, bool wide)
>
> could you please keep original names as dst_reg/src_reg instead of
> reg_rm/reg_reg ?
> reg_reg reads really odd and reg_rm is equally puzzling unless the reader
> studied
> intel's manual. I didn't. All these new abbreviations are challenging for me.
OK. I originally changed it to use the x86 names because in theory you
could do:
maybe_emit_rex(, src_reg, dst_reg);
so the names would look backwards when you jump into the function
implementation.
> > +{
> > + u8 *prog = *pprog;
> > + int cnt = 0;
> > +
> > + if (wide)
>
> what is 'wide' ? Why not to call it 'bool is_alu64' ?
Ack - there's precedent in the file for 'is64' so I'll go with that.
Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte
On Fri, Nov 27, 2020 at 05:57:27PM +, Brendan Jackman wrote:
> The JIT case for encoding atomic ops is about to get more
> complicated. In order to make the review & resulting code easier,
> let's factor out some shared helpers.
>
> Signed-off-by: Brendan Jackman
> ---
> arch/x86/net/bpf_jit_comp.c | 39 ++---
> 1 file changed, 23 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 94b17bd30e00..a839c1a54276 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, u32
> r2, int off)
> *pprog = prog;
> }
>
> +/*
> + * Emit a REX byte if it will be necessary to address these registers
What is "REX byte" ?
May be rename it to maybe_emit_mod() ?
> + */
> +static void maybe_emit_rex(u8 **pprog, u32 reg_rm, u32 reg_reg, bool wide)
could you please keep original names as dst_reg/src_reg instead of
reg_rm/reg_reg ?
reg_reg reads really odd and reg_rm is equally puzzling unless the reader
studied
intel's manual. I didn't. All these new abbreviations are challenging for me.
> +{
> + u8 *prog = *pprog;
> + int cnt = 0;
> +
> + if (wide)
what is 'wide' ? Why not to call it 'bool is_alu64' ?
> + EMIT1(add_2mod(0x48, reg_rm, reg_reg));
> + else if (is_ereg(reg_rm) || is_ereg(reg_reg))
> + EMIT1(add_2mod(0x40, reg_rm, reg_reg));
> + *pprog = prog;
> +}
[PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte
The JIT case for encoding atomic ops is about to get more
complicated. In order to make the review & resulting code easier,
let's factor out some shared helpers.
Signed-off-by: Brendan Jackman
---
arch/x86/net/bpf_jit_comp.c | 39 ++---
1 file changed, 23 insertions(+), 16 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 94b17bd30e00..a839c1a54276 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, u32 r2,
int off)
*pprog = prog;
}
+/*
+ * Emit a REX byte if it will be necessary to address these registers
+ */
+static void maybe_emit_rex(u8 **pprog, u32 reg_rm, u32 reg_reg, bool wide)
+{
+ u8 *prog = *pprog;
+ int cnt = 0;
+
+ if (wide)
+ EMIT1(add_2mod(0x48, reg_rm, reg_reg));
+ else if (is_ereg(reg_rm) || is_ereg(reg_reg))
+ EMIT1(add_2mod(0x40, reg_rm, reg_reg));
+ *pprog = prog;
+}
+
/* LDX: dst_reg = *(u8*)(src_reg + off) */
static void emit_ldx(u8 **pprog, u32 size, u32 dst_reg, u32 src_reg, int off)
{
@@ -854,10 +869,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs,
u8 *image,
case BPF_OR: b2 = 0x09; break;
case BPF_XOR: b2 = 0x31; break;
}
- if (BPF_CLASS(insn->code) == BPF_ALU64)
- EMIT1(add_2mod(0x48, dst_reg, src_reg));
- else if (is_ereg(dst_reg) || is_ereg(src_reg))
- EMIT1(add_2mod(0x40, dst_reg, src_reg));
+ maybe_emit_rex(, dst_reg, src_reg,
+ BPF_CLASS(insn->code) == BPF_ALU64);
EMIT2(b2, add_2reg(0xC0, dst_reg, src_reg));
break;
@@ -1301,20 +1314,16 @@ xadd: emit_modrm_dstoff(,
dst_reg, src_reg, insn->off);
case BPF_JMP32 | BPF_JSGE | BPF_X:
case BPF_JMP32 | BPF_JSLE | BPF_X:
/* cmp dst_reg, src_reg */
- if (BPF_CLASS(insn->code) == BPF_JMP)
- EMIT1(add_2mod(0x48, dst_reg, src_reg));
- else if (is_ereg(dst_reg) || is_ereg(src_reg))
- EMIT1(add_2mod(0x40, dst_reg, src_reg));
+ maybe_emit_rex(, dst_reg, src_reg,
+ BPF_CLASS(insn->code) == BPF_JMP);
EMIT2(0x39, add_2reg(0xC0, dst_reg, src_reg));
goto emit_cond_jmp;
case BPF_JMP | BPF_JSET | BPF_X:
case BPF_JMP32 | BPF_JSET | BPF_X:
/* test dst_reg, src_reg */
- if (BPF_CLASS(insn->code) == BPF_JMP)
- EMIT1(add_2mod(0x48, dst_reg, src_reg));
- else if (is_ereg(dst_reg) || is_ereg(src_reg))
- EMIT1(add_2mod(0x40, dst_reg, src_reg));
+ maybe_emit_rex(, dst_reg, src_reg,
+ BPF_CLASS(insn->code) == BPF_JMP);
EMIT2(0x85, add_2reg(0xC0, dst_reg, src_reg));
goto emit_cond_jmp;
@@ -1350,10 +1359,8 @@ xadd:emit_modrm_dstoff(,
dst_reg, src_reg, insn->off);
case BPF_JMP32 | BPF_JSLE | BPF_K:
/* test dst_reg, dst_reg to save one extra byte */
if (imm32 == 0) {
- if (BPF_CLASS(insn->code) == BPF_JMP)
- EMIT1(add_2mod(0x48, dst_reg, dst_reg));
- else if (is_ereg(dst_reg))
- EMIT1(add_2mod(0x40, dst_reg, dst_reg));
+ maybe_emit_rex(, dst_reg, dst_reg,
+ BPF_CLASS(insn->code) ==
BPF_JMP);
EMIT2(0x85, add_2reg(0xC0, dst_reg, dst_reg));
goto emit_cond_jmp;
}
--
2.29.2.454.gaff20da3a2-goog
