Re: [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for libeth_xdp_tx_xmit_bulk()

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Nathan Chancellor <nathan@kernel.org>
Date: Mon, 27 Oct 2025 13:54:09 -0700

> On Mon, Oct 27, 2025 at 03:59:51PM +0100, Alexander Lobakin wrote:
>> Hmmm,
>>
>> For this patch:
>>
>> Acked-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> 
> Thanks a lot for taking a look, even if it seems like we might not
> actually go the route of working around this.
> 
>> However,
>>
>> The XSk metadata infra in the kernel relies on that when we call
>> xsk_tx_metadata_request(), we pass a static const struct with our
>> callbacks and then the compiler makes all these calls direct.
>> This is not limited to libeth (although I realize that it triggered
>> this build failure due to the way how I pass these callbacks), every
>> driver which implements XSk Tx metadata and calls
>> xsk_tx_metadata_request() relies on that these calls will be direct,
>> otherwise there'll be such performance penalty that is unacceptable
>> for XSk speeds.
> 
> Hmmmm, I am not really sure how you could guarantee that these calls are
> turned direct from indirect aside from placing compile time assertions
> around like this... when you say "there'll be such performance penalty

You mean in case of CFI or in general? Because currently on both GCC and
Clang with both OPTIMIZE_FOR_{SIZE,SPEED} they get inlined in every driver.

> that is unacceptable for XSk speeds", does that mean that everything
> will function correctly but slower than expected or does the lack of
> proper speed result in functionality degredation?

Nothing would break, just work way slower than expected.
xsk_tx_metadata_request() is called for each Tx packet (when Tx metadata
is enabled). Average XSK Tx perf is ~35-40 Mpps (millions of packets per
second), often [much] higher. Having an indirect call there would divide
it by n.

> 
>> Maybe xsk_tx_metadata_request() should be __nocfi as well? Or all
>> the callers of it?
> 
> I would only expect __nocfi_generic to be useful for avoiding a problem
> such as this. __nocfi would be too big of a hammer because it would

Yep, sorry, I actually meant __nocfi_generic...

> cause definite problems if these calls were emitted as indirect ones, as
> they would not have the CFI setup on the caller side, resulting in
> problems that are now flagged by commit 894af4a1cde6 ("objtool: Validate
> kCFI calls") in mainline. It sounds like it could be useful on
> xsk_tx_metadata_request() if we decide to further pursue this series but
> given we could just bump the version of LLVM necessary for CONFIG_CFI on
> ARM, we may just go that route.
> 
> Cheers,
> Nathan

Thanks,
Olek