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

Maciej S. Szmigiero
2023-10-02T16:33:17Z

On 26.09.2023 00:25, Tom Lendacky wrote:
> 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.

So, should I prepare v2 with MSR_AMD64_BU_CFG -> MSR_AMD64_TW_CFG change?

Thanks,
Maciej