Re: [PATCH v11 3/4] x86, mce: Add __mcsafe_copy()

[Luck, Tony]

On Thu, Feb 18, 2016 at 10:12:42AM -0800, Linus Torvalds wrote:
> On Wed, Feb 17, 2016 at 10:20 AM, Tony Luck <tony.luck@xxxxxxxxx> wrote:
> >
> > If we faulted during the copy, then 'trapnr' will say which type
> > of trap (X86_TRAP_PF or X86_TRAP_MC) and 'remain' says how many
> > bytes were not copied.
> 
> So apart from the naming, a couple of questions:
> 
>  - I'd like to see the actual *use* case explained, not just what it does.

First user is libnvdimm. Dan Williams already has code to use this
so that kernel code accessing persistent memory can return -EIO to
a user instead of crashing the system if the cpu runs into an
uncorrected error during the copy.

I would also lkie use this for a machine check aware
copy_from_user() which would avoid crashing the kernel 
when the uncorrected error is in a user page (we can SIGBUS
the user just like we do if the user touched the poison themself).

copy_to_user() is also interesting if the source address is the
page cache. I think we can also avoid crashing the kernel in this
case too - but I haven't thought that all the way through.

>  - why does this use the complex - and slower, on modern machines -
> unrolled manual memory copy, when you might as well just use a single
> 
>      rep ; movsb
> 
>     which not only makes it smaller, but makes the exception fixup trivial.

Because current generation cpus don't give a recoverable machine
check if we consume with a "rep ; movsb" :-(
When we have that we can pick the best copy function based
on the capabilities of the cpu we are running on.

>  - why not make the "bytes remaining" the same as for a user-space
> copy (ie return it as the return value)?
> 
>  - at that point, it ends up looking a *lot* like uaccess_try/catch,
> which gets the error code from current_thread_info()->uaccess_err

For my copy_from_user/copy_to_user cases we need to know both the
number of remaining bytes and also *why* we stopped copying. We
might have #PF, in which case we return -EFAULT to the user, if
we have #MC then the recovery path is different (need to offline
the page, SIGBUS the user, ...)

-Tony