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Re: [Qemu-devel] Announcing qboot, a minimal x86 firmware for QEMU

From: Christopher Covington
Subject: Re: [Qemu-devel] Announcing qboot, a minimal x86 firmware for QEMU
Date: Wed, 27 May 2015 08:50:37 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:36.0) Gecko/20100101 Thunderbird/36.0

On 05/27/2015 05:30 AM, Paolo Bonzini wrote:
> On 26/05/2015 23:25, Christopher Covington wrote:
>> On 05/25/2015 08:53 AM, Paolo Bonzini wrote:
>>> On 22/05/2015 13:12, Daniel P. Berrange wrote:
>>>> In
>>>> particular I don't see why we need to have a SATA controller and ISA/LPC
>>>> bridge in every virt machine - root PCI bus only should be possible, as you
>>>> can provide disks via virtio-blk or virtio-scsi and serial, parallel, 
>>>> mouse,
>>>> floppy via PCI devices and/or by adding a USB bus in the cases where you
>>>> really need one.
>>> I think removing the ISA/LPC bridge is hard.  It includes the real-time
>>> clock and fw_cfg, for example.
>> Could VirtIO specified replacements make sense for these peripherals?
> Not really.  virtio is too heavyweight

I'd be curious to read where in your estimation this weight lies. Is it
one-time initialization or recurring? Is it specific to the PCI transport or
does MMIO suffer from it as well?

> and you'd be reinventing the wheel unnecessarily.

In my mind the utility of peripherals that are instruction set architecture
agnostic and can work over several transports is in reducing the amount of
(emulator/hypervisor, firmware, and OS) code used, and therefore in need of
maintenance, for common system emulation and virtualization use cases.

> For example, ARM's "-M virt" uses a pl011 block for the RTC, and also
> uses fw_cfg.  Another commonly used ISA device is the UART, for which
> again -M virt uses a pl031.

(UART is PL011; RTC is PL031)


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