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

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Date:	Fri, 29 Apr 2016 14:14:02 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	"Richard W.M. Jones" <rjones@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>
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
Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

On 04/29/2016 11:37 AM, 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.
>
> What's the right way to avoid this 25ms delay?
>
> Rich.
>
param && cpu_has_hypervisor?
.... that restricts it to your use case.
... you can 1+ which hypervisor it is if you add export for pv_info(.name).
... all of which only works on x86, as cpu_has_hypervisor is defined here:
                arch/x86/include/asm/cpufeature.h

which points to a better design based on config option:
     if(CONFIG_<NEW_OPTION>) <your optimization of choice>
then it can be optimized in & out as needed.
Single, binary kernel for bare-metal & virt (e.g., rhel) would
bear the additional CONFIG_xxx check, but that's during boot/init,
which isn't perf sensitive on real hw.

You could then use the above CONFIG_NEW_OPTION to optimize other kvm-guest
boot init callbacks/paths.