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As I've made a fair bit of headway since LinaroConnect, I wanted to drop a line on my current progress with porting TianoCore to KVM
Summary (tl;dr version):
KVM can start TianoCore, and boot all the way to shell, and access HDDs via VirtioBlk. We can start grub and successfully retrieve files from ext partitions, load a device tree, and start the kernel. The kernel runs through most of the EFI stub, but falls over during ExitBootServices()
Long Version:
So, after much blood sweat and tears, we're finally at the point of trying to actually start a kernel, though this (for the moment) remains an elusive goal. The current problem is that once we call EBS(), we get an exception from EFI with no Image information, which means the exception handler doesn't know where it came from. After several seconds, we get a second exception from within DxeCore, and then EFI falls over.
Debugging EFI is difficult and error prone, combined with limited debug facilities from the gdb-stub in QEMU (no breakpoints), and no decent way to load all of EFI itself (you have to run add-symbol-file manually with the output of commands printed on the console; supposedly its possible to generate a giant GdbSyms.dll file to import in a single go, but I haven't succeeded at this). This is further complicated that it appears we're asserting somewhere in a driver, and short of adding printfs to *every* driver, its impossible to know which is asseting.
Previous attempts to debug assets shows that EFI does "odd" things to the stack when we hit an exception, making walking it with GDB impossible. I need to figure out what madness EFI does with my SP so I can get the entire stack on an explosion, but this remains at best hopeful thinking.
Further complicating things is that during EBS, my print debugging goes away. I might just cheat and roll a simple assembly function to bang out messages through serial without calling anything else. Ugly as sin, but this should let me get useful debug output through the EBS framework. Complicating matters is that I need to locate each and all EBS() event functions, which are spread *everywhere* in TianoCore, and then debug them each individually.
I'm open to ideas on how best to accomplish this.
On a larger scale, there are a couple of other bugs and odds and ends which currently affect us:
* wfi doesn't work
THis is probably the biggest w.r.t. to functionality that should work, but doesn't. The EFI event loop is built on checking the timer, then calling wfi to check the timer later. The problem here is we call wfi ... and UEFI never comes back despite events firing (I can put print code in the interrupt handler to confirm this). This may be related to the VGIC errors I get running kvm under foundation, but haven't taken the time to properly nail down the bug here.
This was worked around by commenting out the wfi, turning event loop into a busy loop, but this has to be resolved before we can ever consider merging it
* No RTC
I looked through virt.c in KVM, and as best I can tell, I've got no RTC at all (no PL031). It also appears that the kernel can't get RTC as running a kernel gets me a 1970 clock. I'm not sure if this is by design or not, but it causes GetTime() to return EFI_ERROR, and I suspect may be one of the exceptions I'm getting avoid (Shell prints a ton of warnings that GetTime is busted).
* No terminfo support (not ARM specific)
EFI assumes its working against a real console or terminal. As such, it doesn't respect anything like termcap or such, and output gets jumbled due to incorrect escape sequencies and such. This isn't specific to ARM as I get identical behavior with OVMF running on stdio or over telnet. Backspace for instance requires me to type ^H into the console manually
As we expect to have this usable remotely, we need to determine how to handle the terminal and escape sequences properly so we're not printing garbage to the screen. A "dumb" mode might just be the best way to handle this, but something like grub do display fancy graphics.
Ideas welcome.
If anyone wants to play with it, my code is on git, but requires a bit of setup to get running, and you need a patch to KVM to successfully start UEFI at the moment. Michael