Re: [Xen-devel] Is there any limitation on the firmware size in Xen?

[Gary Lin <glin@xxxxxxxx>]

On Thu, May 11, 2017 at 01:23:47AM +1000, Jan Beulich wrote:
> >>> On 10.05.17 at 16:29, <CARNOLD@xxxxxxxx> wrote:
> > I was asked the following question which I am posting to the list.
> > 
> > "
> > My name is Gary Lin, and I am the maintainer of the OVMF package, a UEFI
> > implementation for the virtual machines.
> > 
> > Recently, I was testing an upstream patchset[*] and encountered some
> > problems in Xen. Maybe you can help me or give me some hints.
> > 
> > To be short, the edk2/ovmf upstream is going to increase the firmware
> > size from 2MB to 4MB to fulfill windows HCK, and we have to test
> > different types of VM to make sure the patchset really work. When I was
> > using the 2MB build, my Xen HVM worked as expected and showed the boot
> > menu. However, if I use the 4MB build, I got something like this from
> > "xl dmesg":
> > 
> > (d32)  - CPU0 ... 39-bit phys ... fixed MTRRs ... var MTRRs [1/8] ... done.
> > (d32) Testing HVM environment:
> > (d32)  - REP INSB across page boundaries ... passed
> > (d32)  - GS base MSRs and SWAPGS ... passed
> > (d32) Passed 2 of 2 tests
> > (d32) Writing SMBIOS tables ...
> > (d32) Loading OVMF ...
> > (d32) no BIOS ROM image found
> > (d32) *** HVMLoader bug at hvmloader.c:389
> > (d32) *** HVMLoader crashed.
> > 
> > I tried to trace the code and found that in 
> > libxl__load_hvm_firmware_module()
> > in tools/libxl/libxl_dom.c actully loaded the file and add_module_to_list()
> > in tools/libxc/xc_dom_x86.c was loading a firmware "module" with 4194304
> > bytes. However, when hvmloader loaded "bios_module" with get_module_entry(),
> > modlist is NULL. It seems the firmware data was removed for some reason.
> > 
> > Here are my questions:
> > 
> > 1. Is there any limitation on the firmware size in Xen?
> 
> I can't recall an explicit limit on the hvmloader side, but there are a
> number of other restrictions. I'd suggest adding a few more printf()s
> in relevant hvmloader functions (like get_module_entry()) to learn
> whether the image doesn't get passed, or whether instead hvmloader
> doesn't like where it sits (or whether something else has been
> corrupted).
I added a check in the beginning of get_module_entry() like this:

 if (!modlist) {
     printf("null modlist\n");
 }

then I got "null modlist" right after "xl create -c ovmf.cfg". That's
why I want to know how hvm_start_info is passed to hvmloader.

> 
> > 2. How is hvm_start_info passed to hvmloader?
> 
> A pointer to it is in register EBX. But maybe that's no what you really
> meant o ask?
> 
Ya, I found "mov  %ebx, hvm_start_info\n" in the assembly code, but I
don't know how EBX was set. (from an external program?)

Thanks,

Gary Lin

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