Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths

[Al Viro]

On Wed, Jan 17, 2018 at 11:54:12AM -0800, Dan Williams wrote:
> On Wed, Jan 17, 2018 at 10:52 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Jan 17, 2018 at 02:17:26PM +0000, Alan Cox wrote:
> [..]
> >         Incidentally, what about copy_to_iter() and friends?  They
> > check iov_iter flavour and go either into the "copy to kernel buffer"
> > or "copy to userland" paths.  Do we need to deal with mispredictions
> > there?  We are calling a bunch of those on read()...
> >
> 
> Those should be protected by the conversion of __uaccess_begin to
> __uaccess_begin_nospec that includes the lfence.

Huh?  What the hell does it do to speculative execution of "memcpy those
suckers" branch?