Re: [PATCH bpf-next v2 06/11] bpf, arm64, powerpc: Change nospec to include v1 barrier

[Hari Bathini <hbathini@linux.ibm.com>]

On 18/05/25 4:11 pm, Hari Bathini wrote:
> 
> 
> On 21/04/25 2:47 pm, Luis Gerhorst wrote:
>> This changes the semantics of BPF_NOSPEC (previously a v4-only barrier)
>> to always emit a speculation barrier that works against both Spectre v1
>> AND v4. If mitigation is not needed on an architecture, the backend
>> should set bpf_jit_bypass_spec_v4/v1().
>>
>> As of now, this commit only has the user-visible implication that unpriv
>> BPF's performance on PowerPC is reduced. This is the case because we
>> have to emit additional v1 barrier instructions for BPF_NOSPEC now.
>>
>> This commit is required for a future commit to allow us to rely on
>> BPF_NOSPEC for Spectre v1 mitigation. As of this commit, the feature
>> that nospec acts as a v1 barrier is unused.
>>
>> Commit f5e81d111750 ("bpf: Introduce BPF nospec instruction for
>> mitigating Spectre v4") noted that mitigation instructions for v1 and v4
>> might be different on some archs. While this would potentially offer
>> improved performance on PowerPC, it was dismissed after the following
>> considerations:
>>
>> * Only having one barrier simplifies the verifier and allows us to
>>    easily rely on v4-induced barriers for reducing the complexity of
>>    v1-induced speculative path verification.
> 
> Fair enough.
> 
>>
>> * For the architectures that implemented BPF_NOSPEC, only PowerPC has
>>    distinct instructions for v1 and v4. Even there, some insns may be
>>    shared between the barriers for v1 and v4 (e.g., 'ori 31,31,0' and
>>    'sync'). If this is still found to impact performance in an
>>    unacceptable way, BPF_NOSPEC can be split into BPF_NOSPEC_V1 and
>>    BPF_NOSPEC_V4 later. As an optimization, we can already skip v1/v4
>>    insns from being emitted for PowerPC with this setup if
>>    bypass_spec_v1/v4 is set.
>>
> 
> Agreed.
> 
> Acked-by: Hari Bathini <hbathini@linux.ibm.com>

Applies to v3 too :)
Let me send for v3..

- Hari