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Re: [Xen-devel] frame buffer shared by domains on arch arm linux

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
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

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


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"
disk = [ 'phy:/dev/loop0,xvda,w' ]

Kindly advise what could be the reason?

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