Re: [PATCH bpf-next] bpf,x64: Remove unnecessary check on existence of SSE2

[Jie Meng <jmeng@fb.com>]

On Tue, Oct 04, 2022 at 03:04:20AM +0200, KP Singh wrote:
> On Mon, Oct 3, 2022 at 3:17 AM Jie Meng <jmeng@fb.com> wrote:
> >
> > SSE2 and hence lfence are architectural in x86-64 and no need to check
> > whether they're supported in CPU.
> 
> Why do you say this?
> 
> The Instruction set reference does mention that:
> 
> Exceptions:
> 
> #UD If CPUID.01H:EDX.SSE2[bit 26] = 0
> 
> (undefined instruction when the CPUID.SSE2 bit is unset)
> 
> and also that the CPUID feature flag is SSE2

Many x86 extensions predate x86-64. When they designed x86-64, AMD
decided to make some mandatory (and hence architectural) and SSE2 is
one of them[1]. CMOV, NOPL, PAE, NX etc. are other examples.

These extensions' CPUID flags are still set. If code is to be shared
between x86 and x86-64 one can still check CPUID, but bpf_jit_comp.c
is compiled under x86-64 only so the check is redundant.

There's an example Within kernel code too: arch/x86/lib/copy_user_64.S
uses SSE (sfence) and SSE2 (movnti) instructions and doesn't check
CPUID for their presence.

---
[1] https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels

> >
> > Signed-off-by: Jie Meng <jmeng@fb.com>
> > ---
> >  arch/x86/net/bpf_jit_comp.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index d09c54f3d2e0..b2124521305e 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1289,8 +1289,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
> >
> >                         /* speculation barrier */
> >                 case BPF_ST | BPF_NOSPEC:
> > -                       if (boot_cpu_has(X86_FEATURE_XMM2))
> > -                               EMIT_LFENCE();
> > +                       EMIT_LFENCE();
> >                         break;
> >
> >                         /* ST: *(u8*)(dst_reg + off) = imm */
> > --
> > 2.30.2
> >