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

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Date:	Fri, 29 Apr 2016 08:15:40 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	"Matwey V. Kornilov" <matwey@....msu.ru>,
	Jiri Slaby <jslaby@...e.com>,
	Peter Hurley <peter@...leysoftware.com>,
	andriy.shevchenko@...ux.intel.com, phillip.raffeck@....de,
	anton.wuerfel@....de, yamada.masahiro@...ionext.com,
	valentinrothberg@...il.com, linux-serial@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>, ddutile@...hat.com
Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

On Fri, Apr 29, 2016 at 08:41:14AM +0100, Richard W.M. Jones wrote:
> On Fri, Apr 29, 2016 at 10:01:08AM +0300, Matwey V. Kornilov wrote:
> > 2016-04-29 1:18 GMT+03:00 Richard W.M. Jones <rjones@...hat.com>:
> > > [This is an opinionated patch, mainly for discussion.]
> > >
> > > I'm trying to reduce the time taken in the kernel in initcalls, with
> > > my aim being to reduce the current ~700ms spent in initcalls before
> > > userspace, down to something like 100ms.  All times on my Broadwell-U
> > > laptop, under virtualization.  The purpose of this is to be able to
> > > launch VMs around containers with minimal overhead, like Intel Clear
> > > Containers, but using standard distro kernels and qemu.
> > >
> > > Currently the kernel spends 25ms inspecting the UART that we passed to
> > > it from qemu to find out whether it's an 8250/16550/16550A perhaps
> > > with a non-working FIFO or other quirks.  Well, it isn't -- it's a
> > > working emulated 16550A, with a FIFO and no quirks, and if it isn't,
> > > we should fix qemu.
> > >
> > > So the patch detects if we're running virtualized (perhaps it should
> > > only check for qemu/KVM?) and if so, shortcuts the tests.
> > 
> > Does anybody know, whether it is possible to pass through real
> > hardware serial port to a guest? It seems to be as simple as to pass
> > through an interrupt and memory IO ports.
> 
> In theory it seems like something VFIO could do.  Passing something as
> low performance as a serial port through would seem to make little
> sense though.

Multi-port serial PCI cards are not "low performance" and might be
passed through directly to a guest.

thanks,

greg k-h