--Andy
On Apr 18, 2020, at 12:42 PM, Linus Torvalds torvalds@linux-foundation.org wrote:
On Fri, Apr 17, 2020 at 5:12 PM Dan Williams dan.j.williams@intel.com wrote:
@@ -106,12 +108,10 @@ static __always_inline __must_check unsigned long memcpy_mcsafe(void *dst, const void *src, size_t cnt) { #ifdef CONFIG_X86_MCE
i(static_branch_unlikely(&mcsafe_key))return __memcpy_mcsafe(dst, src, cnt);else
if (static_branch_unlikely(&mcsafe_slow_key))return memcpy_mcsafe_slow(dst, src, cnt);#endif
memcpy(dst, src, cnt);return 0;
return memcpy_mcsafe_fast(dst, src, cnt);}
It strikes me that I see no advantages to making this an inline function at all.
Even for the good case - where it turns into just a memcpy because MCE is entirely disabled - it doesn't seem to matter.
The only case that really helps is when the memcpy can be turned into a single access. Which - and I checked - does exist, with people doing
r = memcpy_mcsafe(&sb_seq_count, &sb(wc)->seq_count, sizeof(uint64_t));to read a single 64-bit field which looks aligned to me.
But that code is incredible garbage anyway, since even on a broken machine, there's no actual reason to use the slow variant for that whole access that I can tell. The macs-safe copy routines do not do anything worthwhile for a single access.
Maybe I’m missing something obvious, but what’s the alternative? The _mcsafe variants don’t just avoid the REP mess — they also tell the kernel that this particular access is recoverable via extable. With a regular memory access, the CPU may not explode, but do_machine_check() will, at very best, OOPS, and even that requires a certain degree of optimism. A panic is more likely.