Re: [PATCH v4 11/11] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

[Leo Yan]

On Tue, Jul 13, 2021 at 10:07:03AM +0300, Adrian Hunter wrote:

[...]

> > +/*
> > + * In the compat mode kernel runs in 64-bit and perf tool runs in 32-bit mode,
> > + * 32-bit perf tool cannot access 64-bit value atomically, which might lead to
> > + * the issues caused by the below sequence on multiple CPUs: when perf tool
> > + * accesses either the load operation or the store operation for 64-bit value,
> > + * on some architectures the operation is divided into two instructions, one
> > + * is for accessing the low 32-bit value and another is for the high 32-bit;
> > + * thus these two user operations can give the kernel chances to access the
> > + * 64-bit value, and thus leads to the unexpected load values.
> > + *
> > + *   kernel (64-bit)                        user (32-bit)
> > + *
> > + *   if (LOAD ->aux_tail) { --,             LOAD ->aux_head_lo
> > + *       STORE $aux_data      |       ,--->
> > + *       FLUSH $aux_data      |       |     LOAD ->aux_head_hi
> > + *       STORE ->aux_head   --|-------`     smp_rmb()
> > + *   }                        |             LOAD $data
> > + *                            |             smp_mb()
> > + *                            |             STORE ->aux_tail_lo
> > + *                            `----------->
> > + *                                          STORE ->aux_tail_hi
> > + *
> > + * For this reason, it's impossible for the perf tool to work correctly when
> > + * the AUX head or tail is bigger than 4GB (more than 32 bits length); and we
> > + * can not simply limit the AUX ring buffer to less than 4GB, the reason is
> > + * the pointers can be increased monotonically (e.g in snapshot mode), whatever
> 
> At least for Intel PT, in snapshot mode, the head is always an offset
> into the buffer, so never more than 4GB for a 32-bit perf tool. So maybe
> leave out "(e.g in snapshot mode)"

Sure, will leave out "(e.g in snapshot mode)".

Thanks,
Leo