Messages in this thread | | | From | Michael Ellerman <> | Subject | Re: [PATCH v1 01/13] perf/core: add union to struct perf_branch_entry | Date | Fri, 17 Sep 2021 22:38:37 +1000 |
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Stephane Eranian <eranian@google.com> writes: > Hi, > > On Fri, Sep 17, 2021 at 12:05 AM Michael Ellerman <mpe@ellerman.id.au> wrote: >> >> Stephane Eranian <eranian@google.com> writes: >> > Hi, >> > >> > Thanks for fixing this in the perf tool. But what about the struct >> > branch_entry in the header? >> >> I'm not sure what you mean. >> >> We can't change the order of the fields in the header, without breaking >> existing userspace on BE systems. >> > Ok, I think I had missed that. You are saying that the > #ifdef (__BIG_ENDIAN_BITFIELD) vs __LITTLE_ENDIAN_BITFIELD > > is only added to kernel-only data structures?
No, we *should* have used __BIG/LITTLE_ENDIAN_BITFIELD for the uapi definition, but we forgot.
>> It's annoying that the bit numbers are different between LE & BE, but I >> think it's too late to change that. >> > I agree. > >> So nothing should change in the branch_entry definition in the header. >> >> My comment on your patch was that adding the union with val, makes it >> easier to misuse the bitfields, because now the values can be accessed >> via the bitfields and also via val, but when using val you have to know >> that the bit numbers differ between BE/LE. >> > Ok, I get it now. We do not need to expose val to user. This is added > for kernel code convenience only.
Yeah. Putting the union with val in the uapi encourages userspace to misuse val to bypass the bitfields, and that risks causing endian bugs.
> But if we keep it in kernel, that may break some other rules about > uapi headers.
I don't follow what you mean there.
We could use #ifdef __KERNEL__ in the uapi header to make the union kernel-only, see below, but it's pretty gross.
struct perf_branch_entry { __u64 from; __u64 to; #ifdef __KERNEL__ union { __u64 val; /* to make it easier to clear all fields */ struct { #endif __u64 mispred:1, /* target mispredicted */ predicted:1,/* target predicted */ in_tx:1, /* in transaction */ abort:1, /* transaction abort */ cycles:16, /* cycle count to last branch */ type:4, /* branch type */ reserved:40; #ifdef __KERNEL__ }; }; #endif };
If we just do the inline I suggested we can clear the flags in a single source line, and the generated code seems fine too, eg:
static inline void clear_perf_branch_entry_flags(struct perf_branch_entry *e) { e->mispred = 0; e->predicted = 0; e->in_tx = 0; e->abort = 0; e->cycles = 0; e->type = 0; e->reserved = 0; }
cheers
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