The memtest86+ now supports some command line parameters:
  console= - serial console can be used for memtest86+ control/display,
             example of usage:
               console=ttyS0
               console=ttyS0,115200
               console=ttyS1,115200n8
             Only ttyS0 and ttyS1 are supported.

  btrace   - enable boot trace.

  maxcpus= - limit number of CPUs to use.

  onepass  - run one pass and exit if there are no errors.

  tstlist= - setup a list of tests to run.

  cpumask= - set a CPU (hexadecimal) mask to select CPU's to use for testing.

You can add memtest86+ command line parameters into grub like you are
used with kernel command line parameters, e.g.:
 linux /memtest86+-VERSION console=ttyS0

Unfortunately this is currently supported only with non-ELF memtest86+ image.
You cannot use the command line parameters with the ELF image, because the
command line handling is currently not supported by grub for ELF images.

If you need to use serial console with ELF image, you need to recompile
memtest86+ and explicitly enable/configure serial line. In such case
edit the config.h in memtest86+ sources the following way:

#define SERIAL_CONSOLE_DEFAULT 1
#define SERIAL_TTY 0
#define SERIAL_BAUD_RATE 9600

and recompile memtest86+. This will explicitly enable serial console and
set it to use ttyS0,9600.

NOTE: explicitly enabled serial console may significantly slow down the memory
testing.