linux-kernel - Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

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Date:	Fri, 29 Apr 2016 08:54:13 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	jslaby@...e.com, peter@...leysoftware.com,
	andriy.shevchenko@...ux.intel.com, phillip.raffeck@....de,
	anton.wuerfel@....de, yamada.masahiro@...ionext.com,
	matwey@....msu.ru, valentinrothberg@...il.com,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	ddutile@...hat.com
Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

On Fri, Apr 29, 2016 at 04:37:57PM +0100, Richard W.M. Jones wrote:
> On Fri, Apr 29, 2016 at 08:16:35AM -0700, Greg KH wrote:
> > On Fri, Apr 29, 2016 at 09:10:06AM +0100, Richard W.M. Jones wrote:
> > > On Thu, Apr 28, 2016 at 03:56:33PM -0700, Greg KH wrote:
> > > > On Thu, Apr 28, 2016 at 11:18:33PM +0100, Richard W.M. Jones wrote:
> > > > > Currently autoconf spends 25ms (on my laptop) testing if the UART
> > > > > exported to it by KVM is an 8250 without FIFO and/or with strange
> > > > > quirks, which it obviously isn't.  Assume it is exported to us by a
> > > > > hypervisor, it's a normal, working 16550A.
> > > > > 
> > > > > Signed-off-by: Richard W.M. Jones <rjones@...hat.com>
> > > > > ---
> > > > >  drivers/tty/serial/8250/8250_port.c | 7 +++++++
> > > > >  1 file changed, 7 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > > > > index 00ad2637..de19924 100644
> > > > > --- a/drivers/tty/serial/8250/8250_port.c
> > > > > +++ b/drivers/tty/serial/8250/8250_port.c
> > > > > @@ -1171,6 +1171,13 @@ static void autoconfig(struct uart_8250_port *up)
> > > > >  	if (!port->iobase && !port->mapbase && !port->membase)
> > > > >  		return;
> > > > >  
> > > > > +	/* Hypervisors always export working 16550A devices. */
> > > > > +	if (cpu_has_hypervisor) {
> > > > > +		up->port.type = PORT_16550A;
> > > > > +		up->capabilities |= UART_CAP_FIFO;
> > > > > +		return;
> > > > > +	}
> > > > 
> > > > Have you audited vmware, virtualbox, and everyone else that provides a
> > > > virtual uart device that it will work properly here?
> > > > 
> > > > qemu isn't all the world :)
> > > 
> > > Attached below is a slightly different approach.  If the user passes a
> > > special flag on the kernel command line then we force 16550A and avoid
> > > the 25ms delay.  Since the user chooses the flag, any concerns about
> > > the behaviour of the hypervisor or use of VFIO should be moot.
> > 
> > No, no more module parameters, that's crazy, what happens when you have
> > 64 serial ports in a system, which one is this option for?
> 
> In this (very special) case, the domain is running under qemu and
> I know it only has one serial port that doesn't need probing.

You are trying to take a generalized kernel and somehow "know" about the
hardware ahead of time it is going to run on.  That seems like two
conflicting requirements, don't you agree?

good luck,

greg k-h