mb/emulation/qemu-q35,qemu-i440fx: Add x86_64 support

* Enable optional x86_64 romstage, postcar and ramstage
* Add Kconfig for x86_64 compilation
* Add documentation for x86 qemu mainboards
* Increase CAR stack as x86_64 uses more than 0x4000 bytes

Working:
* Boots to Linux
* Boots to SeaBIOS
* Drops to protected mode at end of ramstage
* Enumerates PCI devices
* Relocateable ramstage
* SMM

Change-Id: If2f02a95b2f91ab51043d4e81054354f4a6eb5d5
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/29667
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 64 insertions, 0 deletions

diff --git a/Documentation/mainboard/emulation/qemu-i440fx.md b/Documentation/mainboard/emulation/qemu-i440fx.md
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+# qemu i440fx mainboard
+
+## Running coreboot in qemu
+Emulators like qemu don't need a firmware to do hardware init.
+The hardware starts in the configured state already.
+
+The coreboot port allows to test non mainboard specific code.
+As you can easily attach a debugger, it's a good target for
+experimental code.
+
+## coreboot x86_64 support
+coreboot historically runs in 32-bit protected mode, even though the
+processor supports x86_64 instructions (long mode).
+
+The qemu-i440fx mainboard has been ported to x86_64 and will serve as
+reference platform to enable additional platforms.
+
+To enable the support set the Kconfig option ``CONFIG_CPU_QEMU_X86_64=y``.
+
+## Installing qemu
+
+On debian you can install qemu by running:
+```bash
+$ sudo apt-get install qemu
+```
+
+On redhat you can install qemu by running:
+```bash
+$ sudo dnf install qemu
+```
+
+## Running coreboot
+
+### To run the i386 version of coreboot (default)
+Running on qemu-system-i386 will require a 32 bit operating system.
+
+```bash
+qemu-system-i386 -bios build/coreboot.rom -serial stdio -M pc
+```
+
+### To run the experimental x86_64 version of coreboot
+Running on qemu-system-x86_64 allows to run a 32 bit or 64 bit operating system,
+as well as firmware.
+
+```bash
+qemu-system-x86_64 -bios build/coreboot.rom -serial stdio -M pc
+```
+
+## Finding bugs
+To test coreboot's x86 code it's recommended to run on a x86 host and enable KVM.
+It will not only run faster, but is closer to real hardware. If you see the
+following message:
+
+    KVM internal error. Suberror: 1
+    emulation failure
+
+something went wrong. The same bug will likely cause a FAULT on real hardware,
+too.
+
+To enable KVM run:
+
+```bash
+qemu-system-x86_64 -bios build/coreboot.rom -serial stdio -M pc -accel kvm -cpu host
+```