Re: [Xen-devel] GPU passthrough issue when VM is configured with 4G memory

["Hao, Xudong" <xudong.hao@xxxxxxxxx>]

> > > > In addition, we found QEMU uses hardcode 0xe0000000 in pc_init1()
> > as
> > > below.
> > > > Should keep these places handle the consistent mmio hole or not?
> > > >
> > > >     if (ram_size >= 0xe0000000 ) {
> > > >         above_4g_mem_size = ram_size - 0xe0000000;
> > > >         below_4g_mem_size = 0xe0000000;
> > > >     } else {
> > > >         above_4g_mem_size = 0;
> > > >         below_4g_mem_size = ram_size;
> > > >     }
> > >
> > > The guys at Intel sent a couple of patches recently to fix this issue:
> > >
> > > http://marc.info/?l=xen-devel&m=136150317011027
> > > http://marc.info/?l=qemu-devel&m=136177475215360&w=2
> > >
> > > Do they solve your problem?
> >
> > These two patches didn't solve our problem.
> >
> 
> I debugged this issue with above two patches. I want to share some
> information and discuss solution here. This issue is actually caused by that a
> VM has a large pci hole (mmio size) which results in QEMU sets memory
> regions inconsistently with hvmloader (QEMU uses hardcode 0xe0000000 in
> pc_init1 and xen_ram_init). I created a virtual device with 1GB mmio size to
> debug this issue. Firstly, QEMU set memory regions except pci hole region in
> pc_init1() and xen_ram_init(), then hvmloader calculated pci_mem_start as
> 0x80000000, and wrote it to TOM register, which triggered QEMU to update
> pci hole region with 0x80000000 using i440fx_update_pci_mem_hole(). Finally
> the windows 7 VM (configured 8G) crashed with BSOD code 0x00000024. If I
> hardcode in QEMU pc_init1 and xen_ram_init to match hvmloader's. Then the
> problem was gone.
> 
> Althrough above two patches will pass actual pci hole start address to QEMU,
> but it's too late, QEMU pc_init1() and xen_ram_init() already set the other
> memory regions, and obviously the pci hole might overlap with ram regions in
> this case. 

How about update ram region as well when pci memory hole updating?

> So I think hvmloader should setup pci devices and calculate pci hole
> first, then QEMU can map memory regions correctly from the beginning.
> 

Every device may be different bar size, the start address of pci hole will be 
different by hvmloader calculating result, so it still need a channel to report 
this address to qemu.

Thanks,
-Xudong


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