Re: [PATCH v7 3/9] i386: hardcode supported eVMCS version to '1'

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Jun 03, 2021 at 01:48:29PM +0200, Vitaly Kuznetsov wrote:
>> Currently, the only eVMCS version, supported by KVM (and described in TLFS)
>> is '1'. When Enlightened VMCS feature is enabled, QEMU takes the supported
>> eVMCS version range (from KVM_CAP_HYPERV_ENLIGHTENED_VMCS enablement) and
>> puts it to guest visible CPUIDs. When (and if) eVMCS ver.2 appears a
>> problem on migration is expected: it doesn't seem to be possible to migrate
>> from a host supporting eVMCS ver.2 to a host, which only support eVMCS
>> ver.1.
>
> Isn't it possible and safe to expose eVMCS ver.1 to the guest on
> a host that supports ver.2?

We expose the supported range, guest is free to use any eVMCS version in
the range (see below):

>
>> 
>> Hardcode eVMCS ver.1 as the result of 'hv-evmcs' enablement for now. Newer
>> eVMCS versions will have to have their own enablement options (e.g.
>> 'hv-evmcs=2').
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  docs/hyperv.txt       |  2 +-
>>  target/i386/kvm/kvm.c | 16 +++++++++++-----
>>  2 files changed, 12 insertions(+), 6 deletions(-)
>> 
>> diff --git a/docs/hyperv.txt b/docs/hyperv.txt
>> index a51953daa833..000638a2fd38 100644
>> --- a/docs/hyperv.txt
>> +++ b/docs/hyperv.txt
>> @@ -170,7 +170,7 @@ Recommended: hv-frequencies
>>  3.16. hv-evmcs
>>  ===============
>>  The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
>> -enabled, it provides Enlightened VMCS feature to the guest. The feature
>> +enabled, it provides Enlightened VMCS version 1 feature to the guest. The feature
>>  implements paravirtualized protocol between L0 (KVM) and L1 (Hyper-V)
>>  hypervisors making L2 exits to the hypervisor faster. The feature is Intel-only.
>>  Note: some virtualization features (e.g. Posted Interrupts) are disabled when
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index c676ee8b38a7..d57eede5dc81 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
>> @@ -1490,13 +1490,19 @@ static int hyperv_init_vcpu(X86CPU *cpu)
>>          ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0,
>>                                    (uintptr_t)&evmcs_version);
>>  
>> -        if (ret < 0) {
>> -            fprintf(stderr, "Hyper-V %s is not supported by kernel\n",
>> -                    kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc);
>> +        /*
>> +         * KVM is required to support EVMCS ver.1. as that's what 'hv-evmcs'
>> +         * option sets. Note: we hardcode the maximum supported eVMCS version
>> +         * to '1' as well so 'hv-evmcs' feature is migratable even when (and if)
>> +         * ver.2 is implemented. A new option (e.g. 'hv-evmcs=2') will then have
>> +         * to be added.
>> +         */
>> +        if (ret < 0 || (uint8_t)evmcs_version > 1) {
>
> Wait, do you really want to get a fatal error every time, after a
> kernel upgrade?
>

Here, evmcs_version (returned by kvm_vcpu_enable_cap()) represents a
*range* of supported eVMCS versions:

(evmcs_highest_supported_version << 8) | evmcs_lowest_supported_version

Currently, this is 0x101 [1..1] range.

The '(uint8_t)evmcs_version > 1' check here means 'eVMCS v1' is no
longer supported by KVM. This is not going to happen any time soon, but
I can imagine in 10 years or so we'll be dropping v1 so the range (in
theory) can be [10..2] -- which would mean eVMCS ver. 1 is NOT
supported. And we can't proceed then.

> I was expecting this:
>
>   vcpu_evmcs_version = 1; /* hardcoded, but can become configurable later */
>   ...
>   kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0, (uintptr_t)&supported_evmcs_version);
>   if (ret < 0 || supported_evmcs_version < vcpu_evmcs_version) {
>     error_setg(...);
>     return;
>   }
>   cpu->hyperv_nested[0] = vcpu_evmcs_version;
>
>
>> +            error_report("Hyper-V %s verson 1 is not supported by kernel",
>> +                         kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc);
>>              return ret;
>>          }
>> -
>> -        cpu->hyperv_nested[0] = evmcs_version;
>> +        cpu->hyperv_nested[0] = (1 << 8) | 1;
>>      }
>>  
>>      return 0;
>> -- 
>> 2.31.1
>> 

-- 
Vitaly