Re: [Qemu-devel] 3.1.0-rc{0,1} doesn't start

[Paolo Bonzini <pbonzini@redhat.com>]

On 20/11/18 00:00, Eduardo Habkost wrote:
> On Mon, Nov 19, 2018 at 04:55:13PM -0500, Bandan Das wrote:
>> balducci@units.it writes:
>>
>>> hello
>>>
>>> I'm building qemu from source and happily using it since a bit
>>> (2.3.0)
>>>
>>> Since 3.1.0-rc0 (including latest 3.1.0-rc1) I'm no more able to start
>>> qemu, getting:
>>>
>>>     ----8<----
>>>     install:115> qemu
>>>     qemu: error: failed to set MSR 0x10a to 0x0
>>>     qemu: /home/balducci/tmp/install-us-d/qemu-3.1.0-rc1.d/qemu-3.1.0-rc0/target/i386/kvm.c:2185: kvm_put_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
>>>     Aborted
>>>     ---->8----
>>>
>> I believe the check on whether MSR_IA32_ARCH_CAPABILITIES is present is
>> incomplete because it can return 0 for data. Can you try this:
>>
>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index f524e7d929..4878ffb90b 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -2002,14 +2002,9 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
>>  #endif
>>  
>>      /* If host supports feature MSR, write down. */
>> -    if (kvm_feature_msrs) {
>> -        int i;
>> -        for (i = 0; i < kvm_feature_msrs->nmsrs; i++)
>> -            if (kvm_feature_msrs->indices[i] == MSR_IA32_ARCH_CAPABILITIES) {
>> -                kvm_msr_entry_add(cpu, MSR_IA32_ARCH_CAPABILITIES,
>> +    if (kvm_arch_get_supported_msr_feature(kvm_state, MSR_IA32_ARCH_CAPABILITIES)) {
>> +        kvm_msr_entry_add(cpu, MSR_IA32_ARCH_CAPABILITIES,
>>                                env->features[FEAT_ARCH_CAPABILITIES]);
> 
> kvm_arch_get_supported_msr_feature() will return the value of the
> MSR on the host side (kvm/x86.c:kvm_get_msr_feature()).  Having
> it return non-zero doesn't mean KVM's
> svm_set_msr(MSR_IA32_ARCH_CAPABILITIES) will work.
> 
> If the MSR doesn't work on KVM_SET_MSRS, it is not supposed to
> appear on KVM_GET_MSR_INDEX_LIST (even if it appears on
> KVM_GET_MSR_FEATURE_INDEX_LIST).  QEMU must check
> KVM_GET_MSR_INDEX_LIST too before including the MSR on the
> KVM_SET_MSRS call.

Yes, this is a KVM bug.  For 3.1, making it "writable if nonzero" is a
valid workaround, because AMD processors always return 0. It's not the
prettiest thing, but it works.

Paolo