Andrii Nakryiko wrote: > On Tue, Dec 1, 2020 at 9:53 PM John Fastabend <john.fastabend@xxxxxxxxx> wrote: > > > > Yonghong Song wrote: > > > > > > > > > > [...] > > > > > > Great, this means that all existing valid uses of > > > > __sync_fetch_and_add() will generate BPF_XADD instructions and will > > > > work on old kernels, right? > > > > > > That is correct. > > > > > > > > > > > If that's the case, do we still need cpu=v4? The new instructions are > > > > *only* going to be generated if the user uses previously unsupported > > > > __sync_fetch_xxx() intrinsics. So, in effect, the user consciously > > > > opts into using new BPF instructions. cpu=v4 seems like an unnecessary > > > > tautology then? > > > > > > This is a very good question. Essentially this boils to when users can > > > use the new functionality including meaningful return value of > > > __sync_fetch_and_add(). > > > (1). user can write a small bpf program to test the feature. If user > > > gets a failed compilation (fatal error), it won't be supported. > > > Otherwise, it is supported. > > > (2). compiler provides some way to tell user it is safe to use, e.g., > > > -mcpu=v4, or some clang macro suggested by Brendan earlier. > > > > > > I guess since kernel already did a lot of feature discovery. Option (1) > > > is probably fine. > > > > For option (2) we can use BTF with kernel version check. If kernel is > > greater than kernel this lands in we use the the new instructions if > > not we use a slower locked version. That should work for all cases > > unless someone backports patches into an older case. > > Two different things: Clang support detection and kernel support > detection. You are talking about kernel detection, I and Yonghong were > talking about Clang detection and explicit cpu=v4 opt-in. > Ah right, catching up on email and reading the thread backwords I lost the context thanks! So, I live in a dev world where I control the build infrastructure so always know clang/llvm versions and features. What I don't know as well is where the program I just built might be run. So its a bit of an odd question from my perspective to ask if my clang supports feature X. If it doesn't support feature X and I want it we upgrade clang so that it does support it. I don't think we would ever write a program to test the assertion. Anyways thanks. > For kernel detection, if there is an enum value or type that gets > added along the feature, then with CO-RE built-ins it's easy to detect > and kernel dead code elimination will make sure that unsupported > instructions won't trip up the BPF verifier. Still need Clang support > to compile the program in the first place, though. +1 > > If there is no such BTF-based way to check, it is still possible to > try to load a trivial BPF program with __sync_fech_and_xxx() to do > feature detection and then use .rodata to turn off code paths relying > on a new instruction set. Right. > > > > > At least thats what I'll probably end up wrapping in a helper function.
