Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

On Mon, Jun 10, 2019 at 3:59 PM Dave Hansen [off-list ref] wrote:

quoted

We then create PR_MARK_CODE_AS_LEGACY.  The kernel will set the bitmap, but it
is going to be slow.

Slow compared to what?  We're effectively adding one (quick) system call
to a path that, today, has at *least* half a dozen syscalls and probably
a bunch of page faults.  Heck, we can probably avoid the actual page
fault to populate the bitmap if we're careful.  That alone would put a
syscall on equal footing with any other approach.  If the bit setting
crossed a page boundary it would probably win.

quoted

Perhaps we still let the app fill the bitmap?

I think I'd want to see some performance data on it first.

Updating legacy bitmap in user space from kernel requires

long q;

get_user(q, ...);
q |= mask;
put_user(q, ...);

instead of

*p |= mask;

get_user + put_user was quite slow when we tried before.

-- 
H.J.