Documentation: Add known bugs of x86_64 code on real hardware

The bugs happen on real hardware or in qemu with KVM enabled.
The very same code runs on some real devices and it runs in qemu
with KVM disabled.

The bugs are so strange that no root cause could be found yet.

Change-Id: I01050f2e38f92c6b96e3258a5b619aa9ee685acc
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44733
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/arch/x86/index.md b/Documentation/arch/x86/index.md
index d22d31de1a..0e14115db8 100644
--- a/Documentation/arch/x86/index.md
+++ b/Documentation/arch/x86/index.md
@@ -65,3 +65,33 @@ The reference implementation is
 * Test how well CAR works with x86_64 and paging
 * Improve mode switches
 * Test libgfxinit / VGA Option ROMs / FSP
+
+## Known bugs on real hardware
+
+According to Intel x86_64 mode hasn't been validated in CAR environments.
+Until now it could be verified on various Intel platforms and no issues have
+been found.
+
+## Known bugs on KVM enabled qemu
+
+The `x86_64` reference code runs fine in qemu soft-cpu, but has serious issues
+when using KVM mode on some machines. The workaround is to *not* place
+page-tables in ROM, as done in
+[CB:49228](https://review.coreboot.org/c/coreboot/+/49228).
+
+Here's a list of known issues:
+
+* After entering long mode, the FPU doesn't work anymore, including accessing
+  MMX registers. It works fine before entering long mode. It works fine when
+  switching back to protected mode. Other registers, like SSE registers, are
+  working fine.
+* Reading from virtual memory, when the page tables are stored in ROM, causes
+  the MMU to abort the "page table walking" mechanism when the lower address
+  bits of the virtual address to be translated have a specific pattern.
+  Instead of loading the correct physical page, the one containing the
+  page tables in ROM will be loaded and used, which breaks code and data as
+  the page table doesn't contain the expected data. This in turn leads to
+  undefined behaviour whenever the 'wrong' address is being read.
+* Disabling paging in compability mode crashes the CPU.
+* Returning from long mode to compability mode crashes the CPU.
+* Entering long mode crashes on AMD host platforms.