Re: [PATCH v2 bpf-next 11/13] bpf: Add bitwise atomic instructions

[Alexei Starovoitov]

On Fri, Nov 27, 2020 at 09:39:10PM -0800, Yonghong Song wrote:
> 
> 
> On 11/27/20 9:57 AM, Brendan Jackman wrote:
> > This adds instructions for
> > 
> > atomic[64]_[fetch_]and
> > atomic[64]_[fetch_]or
> > atomic[64]_[fetch_]xor
> > 
> > All these operations are isomorphic enough to implement with the same
> > verifier, interpreter, and x86 JIT code, hence being a single commit.
> > 
> > The main interesting thing here is that x86 doesn't directly support
> > the fetch_ version these operations, so we need to generate a CMPXCHG
> > loop in the JIT. This requires the use of two temporary registers,
> > IIUC it's safe to use BPF_REG_AX and x86's AUX_REG for this purpose.
> 
> similar to previous xsub (atomic[64]_sub), should we implement
> xand, xor, xxor in llvm?

yes. please. Unlike atomic_fetch_sub that can be handled by llvm.
atomic_fetch_or/xor/and has to be seen as separate instructions
because JITs will translate them as loop.