Re: frame buffer shared by domains on arch arm linux

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Thu, 22 Jan 2015, Mao Mingy wrote:
> On 21/01/2015 18:51, Stefano Stabellini wrote:
> > On Wed, 21 Jan 2015, Mao Mingy wrote:
> > > I am trying to run the PVHVM guest on omap5 (arm arch) so that I can get
> > > the
> > > frame buffer shared by Domains. my setup are:
> > > 1. run xen4.5 after the u-boot.
> > > 2. start the dom0 (on linux kernel 3.12) on xen.
> > 3.12 is pretty old now
> > 
> > 
> > > 3. run the qemu (from xen 4.5) and HVM backend on xen.
> > Is this the QEMU binary that gets compiled as part of the Xen 4.5 build
> > and started automatically by xl if you configure the VM with a
> > framebuffer?
> Yes. it's compiled from Xen 4.5. exact location @ "
> ./xen_4.5/dist/install/usr/local/lib/xen/bin/qemu-system-i386 "
> > > 4. run the guest domain as "Machine: Dummy Virtual Machine, model:
> > > XENVM-4.5"
> > > 5. try to build a frame buffer on guest domain by
> > >      CONFIG_XEN_FBDEV_FRONTEND = y
> > >      CONFIG_FB_VIRTUAL=y
> > > 
> > > after change to domain checking in the first list of xenfb_init(...) in
> > > xen-fbfront.c from
> > >      if (!xen_pv_domain()) --->    if (!xen_domain())
> > > the /dev/fb0 is built up on guest domain. However, writing to this fb has
> > > no
> > > response from screen.
> > > 
> > > Why xenfb_init limited to PV domain only?
> > No it is not. In fact if you use a more recent Linux kernel, you won't
> > need to make any modifications and it should work out of the box.
> Thanks. Check the latest kernel source. it's already modified to if
> (!xen_domain())
> > > Is the PVHVM mode supported by xen4.5 and linux3.12?
> > We don't call "PVHVM" guests on ARM: on ARM we only have one type of
> > guest. From the Linux kernel perspective, they are HVM domains.
> > 
> > 
> > > Looks at the xen.h at the linux kernel source:
> > > enum xen_domain_type {
> > >      XEN_NATIVE,        /* running on bare hardware    */
> > >      XEN_PV_DOMAIN,        /* running in a PV domain      */
> > >      XEN_HVM_DOMAIN,        /* running in a Xen hvm domain */
> > > };
> > > seems PVHVM is not there yet?
> > In this context PVHVM would just be XEN_PV_DOMAIN.
> > 
> > 
> > > Kindly enlighten me what could be the solution to get the frame buffer
> > > shared
> > > by Domains?
> > It should work out of the box with the latest Linux kernel and Xen 4.5,
> > if you add
> > 
> > vfb=['vnc=1']
> > 
> > to the VM config file.
> Unfortunately, It's still no work after add the vfb=[ 'vnc=1' ]. fb
> open/write/ioctl/close from guest
> linux user space program return no error. However, just the screen do NOT
> display the contents wrote to vfb.
> Here is the configuration file when create the guest linux
> 
> kernel = "/xen/images/zImage"
> memory = "128"
> name = "omap5"
> bootargs = "mem=128M vram=16M xencons=hvc"
> vcpus = 1
> extra ="console=hvc0 xencons=hvc root=/dev/xvda ro"

You might want to use console=tty0 if you want to see the kernel output on the framebuffer.

> disk = [ 'phy:/dev/loop0,xvda,w' ]
> vfb=['vnc=1']
> 
> Kindly advise what could be the reason?

Do you have

CONFIG_XEN_FBDEV_FRONTEND=y
CONFIG_INPUT_XEN_KBDDEV_FRONTEND=y
CONFIG_FRAMEBUFFER_CONSOLE=y

in your guest kernel config?

Could you please post the output of xenstore-ls, executing the command
after creating the VM?