Re: [PATCH v8 3/3] x86, mce: Add __mcsafe_copy()

On Mon, Feb 1, 2016 at 3:10 PM, Tony Luck [off-list ref] wrote:

quoted

The most optimal way of alternatively calling two functions would be
something like this, IMO:

alternative_call(memcpy, __mcsafe_copy, X86_FEATURE_MCRECOVERY,
                 ASM_OUTPUT2("=a" (mcsafe_ret.trapnr), "=d" (mcsafe_ret.remain)),
                 "D" (dst), "S" (src), "d" (len));

I hope I've not messed up the calling convention but you want the inputs
in %rdi, %rsi, %rdx and the outputs in %rax, %rdx, respectively. Just
check the asm gcc generates and do not trust me :)

The other thing you probably would need to do is create our own
__memcpy() which returns struct mcsafe_ret so that the signatures of
both functions match.

Yeah, it is a bit of jumping through hoops but this way we do a CALL
<func_ptr> directly in asm, without any JMPs or NOPs padding the other
alternatives methods add.

But if you don't care about a small JMP and that is not a hot path, you
could do the simpler:

        if (static_cpu_has(X86_FEATURE_MCRECOVERY))
                return __mcsafe_copy(...);

        return memcpy();

which adds a JMP or a 5-byte NOP depending on the X86_FEATURE_MCRECOVERY
setting.

Dan,

What do you want the API to look like at the point you make a call
in the libnvdimm code?  Something like:

        r = nvcopy(dst, src, len);

where the innards of nvcopy() does the check for X86_FEATURE_MCE_RECOVERY?

What is useful to you in the return value? The low level __mcsafe_copy() returns
both a remainder and a trap number. But in your case I don't think you
need the trap
number (if the remaining count is not zero, then there must have been a #MC. #PF
isn't an option for you, right?

RIght, we don't need a trap number just an error.  This is the v1
attempt at integrating mcsafe_copy:

https://lists.01.org/pipermail/linux-nvdimm/2016-January/003869.html

I think the only change needed is to use
static_cpu_has(X86_FEATURE_MCRECOVERY) like so:

+static inline int arch_memcpy_from_pmem(void *dst, const void __pmem *src,
+ size_t n)
+{
+ if (static_cpu_has(X86_FEATURE_MCRECOVERY)) {
+ struct mcsafe_ret ret;
+
+ ret = __mcsafe_copy(dst, (void __force *) src, n);
+ if (ret.remain)
+ return -EIO;
+ return 0;
+ }
+ memcpy(dst, (void __force *) src, n);
+ return 0;
+}

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