Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU_CFG access

Tom Lendacky
2023-09-25T22:25:50Z

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