Re: [PATCH] KVM: SVM: Add support for AMD's OSVW feature in guests

[Avi Kivity]

On 12/27/2011 08:51 AM, Boris Ostrovsky wrote:
> On 12/26/11 08:53, Avi Kivity wrote:
>> On 12/19/2011 07:46 PM, Boris Ostrovsky wrote:
>>> From: Boris Ostrovsky<boris.ostrovsky@xxxxxxx>
>>>
>>> In some cases guests should not provide workarounds for errata even
>>> when the
>>> physical processor is affected. For example, because of erratum 400
>>> on family
>>> 10h processors a Linux guest will read an MSR (resulting in VMEXIT)
>>> before
>>> going to idle in order to avoid getting stuck in a non-C0 state.
>>> This is not
>>> necessary: HLT and IO instructions are intercepted and therefore
>>> there is no
>>> reason for erratum 400 workaround in the guest.
>>>
>>> This patch allows us to present a guest with certain errata as fixed,
>>> regardless of the state of actual hardware.
>>>
>>>
>>> +
>>> +static int svm_handle_osvw(struct kvm_vcpu *vcpu,
>>> +               uint32_t msr, uint64_t *val)
>>> +{
>>> +    struct kvm_cpuid_entry2 *cpuid_entry;
>>> +
>>> +    /* Guest OSVW support */
>>> +    cpuid_entry = kvm_find_cpuid_entry(vcpu, 0x80000001, 0);
>>> +    if (!cpuid_entry || !(cpuid_entry->ecx&  bit(X86_FEATURE_OSVW)))
>>> +        return -1;
>>> +
>>> +    /*
>>> +     * Guests should see errata 400 and 415 as fixed (assuming that
>>> +     * HLT and IO instructions are intercepted).
>>> +     */
>>> +    if (msr == MSR_AMD64_OSVW_ID_LENGTH)
>>> +        *val = (osvw_len>= 3) ? (osvw_len) : 3;
>>> +    else {
>>> +        *val = osvw_status&  ~(6ULL);
>>> +
>>> +        if (osvw_len == 0&&  boot_cpu_data.x86 == 0x10)
>>> +            /* Mark erratum 298 as present */
>>> +            *val |= 1;
>>> +    }
>>> +
>>> +    return 0;
>>> +}
>>
>> Please move this to common code, to support cross-vendor migration.
>
> OK. (Note though that the OSVW registers are typically checked during
> system boot so if you migrate a running guest it is unlikely that it
> will read them again)

It will, if it reboots.   Of course that removes any consistency issues,
but we like to be consistent even if there are no issues.

>>> +    if (cpu_has(&boot_cpu_data, X86_FEATURE_OSVW)) {
>>> +        uint64_t len, status;
>>> +        int err;
>>> +
>>> +        len = native_read_msr_safe(MSR_AMD64_OSVW_ID_LENGTH,&err);
>>> +        if (!err)
>>> +            status = native_read_msr_safe(MSR_AMD64_OSVW_STATUS,
>>> +                        &err);
>>> +
>>> +        spin_lock(&svm_lock);
>>> +        if (err)
>>> +            osvw_status = osvw_len = 0;
>>> +        else {
>>> +            if (len<  osvw_len)
>>> +                osvw_len = len;
>>
>> This implies that if a bit is inside len, then the OS must apply the
>> workaround?
>
>
> Almost: when osvw bit is inside len then OS workaround is applied if
> the bit is set. And if the bit is outside len then OS workaround
> should always be applied.

Ok.

Note that hardware_enable() can be called as part of host cpu hotplug,
so the values can change dynamically.  Not really sure what to do in
this case.

>
>
>>
>>> +            osvw_status |= status;
>>> +            osvw_status&= (1ULL<<  osvw_len) - 1;
>>> +        }
>>> +        spin_unlock(&svm_lock);
>>> +    }
>>> +
>>>       svm_init_erratum_383();
>>>
>>> @@ -3092,6 +3162,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu,
>>> unsigned ecx, u64 data)
>>>       case MSR_VM_IGNNE:
>>>           pr_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n",
>>> ecx, data);
>>>           break;
>>> +    case MSR_AMD64_OSVW_ID_LENGTH:
>>> +    case MSR_AMD64_OSVW_STATUS:
>>> +        /* Writes are ignored */
>>> +        break;
>>>       default:
>>>           return kvm_set_msr_common(vcpu, ecx, data);
>>>       }
>>
>> Best to allow writes, the manual says writes are allowed for bios code,
>> and the OS should just avoid it.
>
> BIOS is usually the one who sets these bits and OS indeed shouldn't
> try to write the registers.
>
> The reason I decided to ignore the writes is for cases when we are on
> a buggy BIOS and a guest writing through to the registers can alter
> system state.

They'd write to a virtual set of MSRs, not the real ones.

>
>>
>> Not sure what to do here about live migration, since the guest will not
>> adjust its behaviour.  Should management software read those MSRs from
>> userspace and check that they're consistent across a cluster?
>>
>
> Either that or start the guest on the most "broken" processor
> (assuming that OSVW registers are only read during boot).
>
> Alternatively the management SW would set emulated values of the MSRs
> for all systems in the cluster but I am not sure whether this is
> supported yet.

If you allow writing, then host userspace can coordinate across the
cluster and use KVM_SET_MSR to set up the most strict values.

-- 
error compiling committee.c: too many arguments to function

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