Re: [Qemu-devel] [Xen-devel] [RFC PATCH 0/2] support xen HVM direct kernel boot

[Chun Yan Liu]

>>> On 6/2/2014 at 11:14 PM, in message
<address@hidden>, Stefano
Stabellini <address@hidden> wrote: 
> On Fri, 30 May 2014, Konrad Rzeszutek Wilk wrote: 
> > On Thu, May 29, 2014 at 11:23:22AM +0800, Chunyan Liu wrote: 
> > > Following previous discussion: 
> > > https://lists.gnu.org/archive/html/qemu-devel/2014-04/msg03843.html 
> > >  
> > > I spent lot of time to understand xen xc_hvm_build, hvmloader code, 
> > > and qemu pc_memory_init, xen_ram_init code, and wrote experimental 
> > > code to test. Now it's basically working based on seabios. 
> >  
> > Excellent! 
> > >  
> > > Talking about qemu linuxboot.bin/multiboot.bin first, they are 
> > > implemented to intercept boot process, originally by override int19, 
> > > later changed to provide BEV entry which will be called by seabios 
> > > if option rom is selected for booting. linuxboot.bin/multiboot.bin 
> > > will load kernel/initrd and jump to execute kernel directly. 
> > >  
> > > In qemu load_linux() code, it first read out kernel/initrd file and 
> > > store related ADDRs to fw_cfg (from where linuxboot.bin/multiboot.bin 
> > > will get the ADDRs and load kernel/initrd data), then add 
> > > linuxboot.bin/multiboot.bin to option rom with bootindex=0. 
> > >  
> > > In seabios side, it could load and execute ROMs by itself, and according 
> > > to boot order, it will boot from the BEV entry on the option rom. That 
> > > will then be taken over by linuxboot.bin/multiboot.bin. 
> > >  
> > > So, in theory, qemu load_linux() code almost does the work. I just tried 
> > > to reuse it for xen. 
> > >  
> > > xen side patch: pass kernel/initrd/append parameters to qemu-dm 
> > > qemu side patch: reuse load_linux() for xen hvm direct kernel boot. 
> > >     Different from pc_memory_init which does lots of ram alloc work 
> > >     and rom/bios loading work, for xen, we only need to init a basic 
> > >     fw_cfg device used by load_linux() to store ADDRs and 
> > >     linuxboot.bin/multiboot.bin to retrive ADDRs, then load_linux(), 
> > >     after that, do real add option rom work to add 
> > >     linuxboot.bin/multiboot.bin to system option rom. Other things 
> > >     would be done by seabios smoothly. 
> > >  
> > > In my testing, it's working based on seabios. 
> > > Rombios case is not working. But now that xen prefers qemu upstream, 
> > > seabios will be default, do we need to put effort to make rombios work? 
> >  
> > No. But the code (libxl) should detect whether you are using the 
> > traditional QEMU - which would be using ROM BIOS. If it detects that 
> > it shouldn't allow you to pass the arguments. 
> >  
> > You probably will also need to an docs/ change to document these 
> > new parameters. 
> > > And stubdom case, since I didn't make my guest started even without 
> > > direct 
> > > kernel boot, I don't know if it works. And I still could not well 
> > > understand about the differences to stubdom and non stubdom this work 
> > > may cover. Any problems please point out. 
> >  
> > Hmm, you would have to somehow 'transfer' the images from one domain 
> > (the control domain) to the stubdomain which would do the execution. 
> >  
> > At least the multiboot.bin image. 
>  
> Yeah, unfortunately the approach of implemeting direct kernel boot for 
> HVM guests via QEMU wouldn't work for stubdomains. 
>  
> If we want this to work with stubdoms, it would be best to reimplement 
> kernel and initrd loading in libxl/libxc. 

Thanks. Rouphly thinking, the implementation might be totally different.
For seabios, could load kernel/initrd (and linuxboot.bin) in libxc, and in
qemu find a way to get data and continue with existing code logic (qemu
side changes may be not very clean). And stubdom-dm uses
qemu-xen-traditional, that means rombios by default, that method doesn't
work. Maybe need to change hvmloader? I'll have a look.

> I have to admit that given the extreme simplicity of this patch series, 
> I wouldn't be against merging it, even if it cannot be made to work with 
> stubdoms.  However like in the rombios case, libxl would need to 
> understand that this feature doesn't work with stubdoms and print an 
> appropriate message. 

I'll update printing messages. But now, as you prefer, should I update
existing patch series to let it be in good shape for merge, or should I
abandon it and reimplement to try to make it work with stubdom?

- Chunyan



>  
> > > Personally I'm not quite familiar with the details in this part of code, 
> > > so there might be things not considered, or there might be better method. 
> > > I'm very willing to hear suggestions to improve the work. Any comment or 
> > > feedback would be very appreciated! Thanks. 
> > >   
> > > Chunyan Liu (2): 
> > >   xen: pass kernel initrd to qemu 
> > >   qemu: support xen hvm direct kernel boot 
> > >  
> > > _______________________________________________ 
> > > Xen-devel mailing list 
> > > address@hidden 
> > > http://lists.xen.org/xen-devel 
> >  
>  
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