Re: [PATCH bpf-next] bpf, x86: Use global buffer for trampoline size generation

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Sep 07, 2026 at 12:56:33PM -0700, Alexei Starovoitov wrote:
> On Mon Sep 7, 2026 at 9:05 AM PDT, Jiri Olsa wrote:
> > Currently arch_bpf_trampoline_size allocates and frees a temporary
> > trampoline buffer on every invocation. The buffer is only used as a
> > scratch space while __arch_prepare_bpf_trampoline() calculates the
> > required size, and the generated trampoline is discarded.
> >
> > Allocating a writable scratch page during kernel initialization and
> > reusing it for all size calculations. This improves tracing_multi
> > attachment time.
> >
> > With current code:
> >
> >   # ./test_progs -t tracing_multi_bench_attach -v
> >   ...
> >   serial_test_tracing_multi_bench_attach: found 55227 functions
> >   serial_test_tracing_multi_bench_attach: attached in   1.563s
> >   serial_test_tracing_multi_bench_attach: detached in   0.256s
> >
> > With the fix:
> >
> >   # ./test_progs -t tracing_multi_bench_attach -v
> >   ...
> >   serial_test_tracing_multi_bench_attach: found 55235 functions
> >   serial_test_tracing_multi_bench_attach: attached in   0.798s
> >   serial_test_tracing_multi_bench_attach: detached in   0.258s
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > was "bpf, x86: Add support for jit dry run",
> > - doing this by having single scratch page instead as suggested by Alexei
> >
> >  arch/x86/net/bpf_jit_comp.c | 30 +++++++++++++++---------------
> >  1 file changed, 15 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index bba351944202..13ef0d53ca29 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -9,6 +9,7 @@
> >  #include <linux/filter.h>
> >  #include <linux/if_vlan.h>
> >  #include <linux/bitfield.h>
> > +#include <linux/init.h>
> >  #include <linux/bpf.h>
> >  #include <linux/bpf_verifier.h>
> >  #include <linux/memory.h>
> > @@ -35,6 +36,15 @@ void __asan_store8(void *p);
> >  
> >  static bool all_callee_regs_used[4] = {true, true, true, true};
> >  
> > +static void *trampoline_size_image;
> > +
> > +static int __init init_trampoline_size_image(void)
> > +{
> > +	trampoline_size_image = execmem_alloc(EXECMEM_MODULE_DATA, PAGE_SIZE);
> 
> I think I asked it earlier... why does it have to be execmem ?
> Can it be normal page?

ugh sorry I forgot.. the page/image needs to be in execmem range
for emitting call/jmp otherwise the delta won't fit in 4 bytes
and it fails on emit_patch is_simm32 check

> 
> If so then alloc it and free it every time. No need to keep one page in reserve.

hum, you mean drop the change then?

jirka