Re: [PATCH bpf-next v5 1/4] bpf: add bpf_get_cpu_cycles kfunc

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2024-11-12 at 13:53 -0800, Eduard Zingerman wrote:
> On Tue, 2024-11-12 at 21:39 +0000, Vadim Fedorenko wrote:
> 
> [...]
> 
> > > > +			if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
> > > > +			    imm32 == BPF_CALL_IMM(bpf_get_cpu_cycles)) {
> > > > +				/* Save RDX because RDTSC will use EDX:EAX to return u64 */
> > > > +				emit_mov_reg(&prog, true, AUX_REG, BPF_REG_3);
> > > > +				if (boot_cpu_has(X86_FEATURE_LFENCE_RDTSC))
> > > > +					EMIT_LFENCE();
> > > > +				EMIT2(0x0F, 0x31);
> > > > +
> > > > +				/* shl RDX, 32 */
> > > > +				maybe_emit_1mod(&prog, BPF_REG_3, true);
> > > > +				EMIT3(0xC1, add_1reg(0xE0, BPF_REG_3), 32);
> > > > +				/* or RAX, RDX */
> > > > +				maybe_emit_mod(&prog, BPF_REG_0, BPF_REG_3, true);
> > > > +				EMIT2(0x09, add_2reg(0xC0, BPF_REG_0, BPF_REG_3));
> > > > +				/* restore RDX from R11 */
> > > > +				emit_mov_reg(&prog, true, BPF_REG_3, AUX_REG);
> > > 
> > > Note: The default implementation of this kfunc uses __arch_get_hw_counter(),
> > >        which is implemented as `(u64)rdtsc_ordered() & S64_MAX`.
> > >        Here we don't do `& S64_MAX`.
> > >        The masking in __arch_get_hw_counter() was added by this commit:
> > >        77750f78b0b3 ("x86/vdso: Fix gettimeofday masking").
> > 
> > I think we already discussed it with Alexey in v1, we don't really need
> > any masking here for BPF case. We can use values provided by CPU
> > directly. It will never happen that within one BPF program we will have
> > inlined and non-inlined implementation of this helper, hence the values
> > to compare will be of the same source.
> > 
> > >        Also, the default implementation does not issue `lfence`.
> > >        Not sure if this makes any real-world difference.
> > 
> > Well, it actually does. rdtsc_ordered is translated into `lfence; rdtsc`
> > or `rdtscp` (which is rdtsc + lfence + u32 cookie) depending on the cpu
> > features.
> 
> I see the following disassembly:
> 
> 0000000000008980 <bpf_get_cpu_cycles>:
> ; {
>     8980: f3 0f 1e fa                   endbr64
>     8984: e8 00 00 00 00                callq   0x8989 <bpf_get_cpu_cycles+0x9>
>                 0000000000008985:  R_X86_64_PLT32       __fentry__-0x4
> ;       asm volatile(ALTERNATIVE_2("rdtsc",
>     8989: 0f 31                         rdtsc
>     898b: 90                            nop
>     898c: 90                            nop
>     898d: 90                            nop
> ;       return EAX_EDX_VAL(val, low, high);
>     898e: 48 c1 e2 20                   shlq    $0x20, %rdx
>     8992: 48 09 d0                      orq     %rdx, %rax
>     8995: 48 b9 ff ff ff ff ff ff ff 7f movabsq $0x7fffffffffffffff, %rcx # imm = 0x7FFFFFFFFFFFFFFF
> ;               return (u64)rdtsc_ordered() & S64_MAX;
>     899f: 48 21 c8                      andq    %rcx, %rax
> ;       return __arch_get_hw_counter(1, NULL);
>     89a2: 2e e9 00 00 00 00             jmp     0x89a8 <bpf_get_cpu_cycles+0x28>
> 
> Is it patched when kernel is loaded to replace nops with lfence?
> By real-world difference I meant difference between default
> implementation and inlined assembly.

Talked with Vadim off-list, he explained that 'rttsc nop nop nop' is
indeed patched at kernel load. Regarding S64_MAX patching we just hope
this should never be an issue for BPF use-case.
So, no more questions from my side.

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