Re: [SeaBIOS] [PATCH 2/2] Boot Linux using QEMU fw_cfg DMA interface

[Marc Marí]

On Thu, 6 Aug 2015 13:38:33 +0100
Stefan Hajnoczi  wrote:
> On Thu, Aug 6, 2015 at 12:02 PM, Marc Marí  wrote:
> >  case IPL_TYPE_BEV:
> >    boot_rom(ie->vector);
> >    if (qemu_cfg_dma_enabled()) {
> >        boot_linux_cfg_dma();
> >    } else {
> >        boot_rom(ie->vector);
> >    }
> >    break;
>
> I thought the fw_cfg DMA interface is equivalent to the PIO interface?
>
> Therefore, your Linux loading code completely replaces the
> linuxboot.rom.  Why would you want to keep using linuxboot.rom in the
> PIO case if SeaBIOS now has to code to launch the kernel?
>
> By the way, I think this change is incorrect.  We shouldn't hijack the
> BEV.  Instead fw_cfg should simply add a higher priority IPL entry
> when the kernel file is present.  That way we just don't execute the
> option ROM.

linuxboot.rom is doing all the work of reading and copying the kernel
to the correct position through the PIO interface, in assembly. If we
want to speed up the kernel copying with DMA, it is necessary to change
it, and implementing fw_cfg DMA in C is a lot easier.

This new boot is independent of PIO or DMA, so now linuxboot.rom would
not be necessary. I don't know if this is desirable.

In any case, I agree with adding a new IPL.

Thanks
Marc