Re: [PATCH bpf-next v2 05/15] bpf: Allow storing percpu kptr in map

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: bpf@vger.kernel.org, "Hao Luo" <haoluo@google.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>
Subject: Re: [PATCH bpf-next v2 05/15] bpf: Allow storing percpu kptr in map
Date: Sat, 19 Mar 2022 11:30:28 -0700	[thread overview]
Message-ID: <20220319183028.pwzaoz2qogek6nwz@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20220317115957.3193097-6-memxor@gmail.com>

On Thu, Mar 17, 2022 at 05:29:47PM +0530, Kumar Kartikeya Dwivedi wrote:
> Make adjustments to the code to allow storing percpu PTR_TO_BTF_ID in a
> map. Similar to 'kptr_ref' tag, a new 'kptr_percpu' allows tagging types
> of pointers accepting stores of such register types. On load, verifier
> marks destination register as having type PTR_TO_BTF_ID | MEM_PERCPU |
> PTR_MAYBE_NULL.
> 
> Cc: Hao Luo <haoluo@google.com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  include/linux/bpf.h   |  3 ++-
>  kernel/bpf/btf.c      | 13 ++++++++++---
>  kernel/bpf/verifier.c | 26 +++++++++++++++++++++-----
>  3 files changed, 33 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 702aa882e4a3..433f5cb161cf 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -161,7 +161,8 @@ enum {
>  };
>  
>  enum {
> -	BPF_MAP_VALUE_OFF_F_REF = (1U << 0),
> +	BPF_MAP_VALUE_OFF_F_REF    = (1U << 0),
> +	BPF_MAP_VALUE_OFF_F_PERCPU = (1U << 1),

What is the use case for storing __percpu pointer into a map?