Messages in this thread | | | Subject | Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs. | From | Don Dutile <> | Date | Fri, 29 Apr 2016 14:14:02 -0400 |
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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@redhat.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.
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