linux-kernel - Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU

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Date:   Mon, 25 Sep 2023 20:53:16 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU_CFG access

On 25.09.2023 20:30, Sean Christopherson wrote:
> On Mon, Sep 25, 2023, Maciej S. Szmigiero wrote:
>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>
>> 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.

>> Give this MSR the same treatment that commit 2e32b7190641
>> ("x86, kvm: Add MSR_AMD64_BU_CFG2 to the list of ignored MSRs") gave
>> MSR_AMD64_BU_CFG2 under justification that this MSR is baremetal-relevant
>> only.
> 
> Ugh, that commit set a terrible example.  The kernel change should have been
> conditioned on !X86_FEATURE_HYPERVISOR if the MSR only has meaning for bare metal.

You are right with respect to the original guest kernel change that
triggered the later KVM commit ignoring MSR_AMD64_BU_CFG2.

This doesn't help Windows guests, however.

>> Although apparently it was then needed for Linux guests, not Windows as in
>> this case.
>>
>> With this change, the aforementioned guest setup is able to finish booting
>> successfully.
>>
>> This issue can be reproduced either on a Summit Ridge Ryzen (with
>> just "-cpu host") or on a Naples EPYC (with "-cpu host,stepping=1" since
>> EPYC is ordinarily stepping 2).
> 
> This seems like it needs to be tagged for stable?

Like with just "Cc: stable@...r.kernel.org", but without "Fixes:" tag?
Can do.

>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
>> ---
>>   arch/x86/include/asm/msr-index.h | 1 +
>>   arch/x86/kvm/x86.c               | 2 ++
>>   2 files changed, 3 insertions(+)
>>
>> 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.

>>   #define MSR_AMD64_DE_CFG		0xc0011029
>>   #define MSR_AMD64_DE_CFG_LFENCE_SERIALIZE_BIT	 1
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 9f18b06bbda6..2f3cdd798185 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3639,6 +3639,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   	case MSR_IA32_UCODE_WRITE:
>>   	case MSR_VM_HSAVE_PA:
>>   	case MSR_AMD64_PATCH_LOADER:
>> +	case MSR_AMD64_BU_CFG:
> 
> I am sorely tempted to say that this should be solved in userspace via MSR
> filtering.  IIUC, the MSR truly is model specific, and I don't love the idea of
> effectively ignoring accesses to unknown MSRs.  And I really, really don't want
> KVM to pivot on FMS.
> 
> Paolo, is punting to userspace reasonable, or should we just bite the bullet in
> KVM and commit to ignoring MSRs like this?
> 

Waiting for Paolo's decision here then.

Thanks,
Maciej