On 9/25/23 14:16, Sean Christopherson wrote:
+Tom
On Mon, Sep 25, 2023, Maciej S. Szmigiero wrote:
On 25.09.2023 20:30, Sean Christopherson wrote:
Hyper-V enabled Windows Server 2022 KVM VM cannot be started on Zen1 Ryzen
since it crashes at boot with SYSTEM_THREAD_EXCEPTION_NOT_HANDLED +
STATUS_PRIVILEGED_INSTRUCTION (in other words, because of an unexpected #GP
in the guest kernel).
This is because Windows tries to set bit 8 in MSR_AMD64_BU_CFG and can't
handle receiving a #GP when doing so.
Any idea why?
I guess it is trying to set some chicken bit?
By the way, I tested Windows Server 2019 now - it has the same problem.
So likely Windows 11 and newer version of Windows 10 have it, too.
...
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 1d111350197f..c80a5cea80c4 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -553,6 +553,7 @@
#define MSR_AMD64_CPUID_FN_1 0xc0011004
#define MSR_AMD64_LS_CFG 0xc0011020
#define MSR_AMD64_DC_CFG 0xc0011022
+#define MSR_AMD64_BU_CFG 0xc0011023
What document actually defines this MSR? All of the PPRs I can find for Family 17h
list it as:
MSRC001_1023 [Table Walker Configuration] (Core::X86::Msr::TW_CFG)
It's partially documented in various AMD BKDGs, however I couldn't find
any definition for this particular bit (8) - other than that it is reserved.
I found it as MSR_AMD64_BU_CFG for Model 16h, but that's Jaguar/Puma, not Zen1.
My guess is that Windows is trying to write this thing:
MSRC001_1023 [Table Walker Configuration] (Core::X86::Msr::TW_CFG)
Read-write. Reset: 0000_0000_0000_0000h.
_lthree0_core[3,1]; MSRC001_1023
Bits Description
63:50 Reserved.
49 TwCfgCombineCr0Cd: combine CR0_CD for both threads of a core. Read-write. Reset: 0. Init: BIOS,1.
1=The host Cr0_Cd values from the two threads are OR'd together and used by both threads.
48:0 Reserved.
Though that still doesn't explain bit 8... Perhaps a chicken-bit related to yet
another speculation bug?
Boris or Tom, any idea what Windows is doing? I doubt it changes our options in
terms of "fixing" this in KVM, but having a somewhat accurate/helpful changelog
would be nice.
It's definitely not related to a speculation bug, but I'm unsure what was
told to Microsoft that has them performing that WRMSR. The patch does the
proper thing, though, as a guest shouldn't be updating that setting.
And TW_CFG is the proper name of that MSR for Zen.
Thanks,
Tom
