Re: Fbdev graphics broken in xen/next dom0

[Eamon Walsh <ewalsh@tycho.nsa.gov>]

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On 03/12/2010 04:42 PM, Jeremy Fitzhardinge wrote:
> On 03/12/2010 12:24 PM, Eamon Walsh wrote:
>   
>> I work in the same group as Dave Quigley and George Coker.  I'm working
>> on a graphical switcher application for Xen which uses the DirectFB
>> library on top of Linux VESA fbdev.  This runs in dom0 at the moment.
>> I'm using the latest xen/next pvops dom0 and xen-unstable hypervisor
>> compiled from source, with vga=ask so I can boot dom0 in a graphical mode.
>>
>> The problem I'm having is illustrated by the attached test program that
>> displays a green background with a white square for 10 seconds when run
>> as root.  It doesn't work on the xen/next / xen-unstable combo.  The
>> program runs and exits normally but all I see is a black screen.
>>
>> The program *does* work on xen/next running on the bare metal.  It also
>> works using the xen-unstable hypervisor with an older dom0, the 2.6.31.4
>> kernel with Novell patches.  So I think the issue is in the xen/next
>> kernel.  I've run the test program on different machines and observed
>> the same behavior.
>>
>> The xen-unstable / 2.6.31.4 dom0 combination works and I'm using that
>> for the moment but I'd like to be using pvops.  I would be happy to run
>> more tests / provide more data if needed.
>>    
>>     
> What's the hardware?  Do any messages appear either on the dom0 console 
> or the Xen console?  Does booting with a vga console help?
>   

The hardware is a Dell Latitude E6500 with nvidia graphics.  I also see
the issue on a Dell Optiplex 960 desktop with Intel graphics.  No
obvious messages on the consoles.  I am booting in VGA mode.

I have narrowed the problem down: it has something to do with mmap of
/dev/fb0 not syncing.  The attached C code mmaps /dev/fb0 and writes
some random bits.  On a configuration that does work (2.6.31.4 on
4.0-rc6, or xen/next on bare metal) the random bits are visible on the
screen.  With xen/next on 4.0-rc6, nothing is visible.  Calling msync()
before the sleep has no effect.  Also, using write() on /dev/fb0 always
works so it appears to be mmap related.


-- 

Eamon Walsh 
National Security Agency


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#include <sys/types.h>
#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

static void bail(int cond, const char *msg)
{
	if (cond) {
		perror(msg);
		exit(1);
	}
}

int main(void)
{
	int id = open("/dev/urandom", O_RDONLY), fd = open("/dev/fb0", O_RDWR);
	unsigned *addr;
	char buf[4096];

	bail(fd < 0, "/dev/fb0");
	bail(id < 0, "/dev/urandom");

	addr = mmap(NULL, sizeof(buf), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
	bail(addr == MAP_FAILED, "mmap");

	read(id, buf, sizeof(buf));
	memcpy(addr, buf, sizeof(buf));
	// write(fd, buf, sizeof(buf));

	sleep(10);

	close(id);
	close(fd);

	return 0;
}

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