> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Gerd Hoffmann
> Sent: 16 May 2006 15:44
> To: Xen devel list
> Subject: [Xen-devel] VT/ioemu: vga memory access?
>
> Hi,
>
> How is vga vram access handled in the device model? Is there 
> some kind of notification system, by mapping those pages 
> read-only, then trap and forward any write access to qemu-dm?
Actually, xen HVM handles all memory mapped IO in the same way - pages are not 
present, causing a page-fault and then checking the address against a "memory 
mapped IO range" in the function mmio_space() [I haven't looked inside this 
function], and if it's a match it's passed to QEMU via handle_mmio(). 
Note also that if paging isn't enabled (real-mode or some other similar 
situation), any page-fault is unconditonally dealt with by calling 
handle_mmio() without checking if it's a MMIO address - because nothing else 
should give a page-fault in non-paging mode.
>
> I'm seeing obscure crashes in vga text mode, looks like they 
> are triggered by a memmove in vga vram, at least this is what 
> xenctx prints me:
>
>
> master-xen root /vm/hvm# /usr/lib/xen/bin/xenctx 35
> eip: c01a59a9
> esp: cf2dbe58
> eax: c00b99a0 ebx: c00b99a0 ecx: fffff661 edx: c00b9860
> esi: c00b8ec0 edi: c00b9000 ebp: c1207000
> cs: 00000060 ds: 0000007b fs: 00000000 gs: 00000033
>
> Stack:
> failed to map PT
> failed to map page.
>
>
> EIP c01a59a9 points into memmove (linux kernel):
>
> c01a5990 <memmove>:
> c01a5990: 57 push %edi
> c01a5991: 39 d0 cmp %edx,%eax
> c01a5993: 56 push %esi
> c01a5994: 53 push %ebx
> c01a5995: 89 c3 mov %eax,%ebx
> c01a5997: 73 07 jae c01a59a0 <memmove+0x10>
> c01a5999: e8 ca ff ff ff call c01a5968 <memcpy>
> c01a599e: eb 0c jmp c01a59ac <memmove+0x1c>
> c01a59a0: 8d 74 0a ff lea 
> 0xffffffff(%edx,%ecx,1),%esi
> c01a59a4: 8d 7c 08 ff lea 
> 0xffffffff(%eax,%ecx,1),%edi
> c01a59a8: fd std
> c01a59a9: f3 a4 repz movsb 
> %ds:(%esi),%es:(%edi)
> ^^^^^^^^^^^^^^^^ here
> c01a59ab: fc cld
> c01a59ac: 89 d8 mov %ebx,%eax
> c01a59ae: 5b pop %ebx
> c01a59af: 5e pop %esi
> c01a59b0: 5f pop %edi
> c01a59b1: c3 ret
>
>
> Note that the edi register points to a page boundary and ecx 
> looks bogous. Also note that "xm unpause", then xenctx again 
> prints the very same register dump, feels like someone 
> handling a fault incorrectly, leading to the very same fault 
> instantly ...
>
> Idea anyone what this might be?
It looks like the length for memmove has been calculated incorrectly (negative 
number), and that would move aroung 4GB of memory. 
I can't really explain why b9000 shouldn't be a valid VGA memory page tho'. 
Perhaps it's because the mode of graphics you're in, and that doesn't allow 
more than 4KB of display memory - I'm surprised about that tho'. 
So it's weird that it's haning there... 
--
Mats 
>
> cheers,
>
> Gerd
>
>
> --
> Gerd Hoffmann <kraxel@xxxxxxx>
> Erst mal heiraten, ein, zwei Kinder, und wenn alles läuft 
> geh' ich nach drei Jahren mit der Familie an die Börse.
> http://www.suse.de/~kraxel/julika-dora.jpeg 
>
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