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

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Message-ID: <20160429074114.GC3826@redhat.com>
Date:	Fri, 29 Apr 2016 08:41:14 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	"Matwey V. Kornilov" <matwey@....msu.ru>
Cc:	Greg KH <gregkh@...uxfoundation.org>, 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 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.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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