Eduardo Habkost <ehabk...@redhat.com> writes:
> On Thu, Jun 03, 2021 at 01:48:34PM +0200, Vitaly Kuznetsov wrote:
>> When Hyper-V SynIC is enabled, we may need to allow Windows guests to make
>> hypercalls (POST_MESSAGES/SIGNAL_EVENTS). No issue is currently observed
>> because KVM is very permissive, allowing these hypercalls regarding of
>> guest visible CPUid bits.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com>
>> ---
>> target/i386/kvm/hyperv-proto.h | 6 ++++++
>> target/i386/kvm/kvm.c | 6 ++++++
>> 2 files changed, 12 insertions(+)
>>
>> diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
>> index e30d64b4ade4..5fbb385cc136 100644
>> --- a/target/i386/kvm/hyperv-proto.h
>> +++ b/target/i386/kvm/hyperv-proto.h
>> @@ -38,6 +38,12 @@
>> #define HV_ACCESS_FREQUENCY_MSRS (1u << 11)
>> #define HV_ACCESS_REENLIGHTENMENTS_CONTROL (1u << 13)
>>
>> +/*
>> + * HV_CPUID_FEATURES.EBX bits
>> + */
>> +#define HV_POST_MESSAGES (1u << 4)
>> +#define HV_SIGNAL_EVENTS (1u << 5)
>> +
>> /*
>> * HV_CPUID_FEATURES.EDX bits
>> */
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index a3897d4d8788..6a32d43e6ec1 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
>> @@ -1343,6 +1343,12 @@ static int hyperv_fill_cpuids(CPUState *cs,
>> /* Unconditionally required with any Hyper-V enlightenment */
>> c->eax |= HV_HYPERCALL_AVAILABLE;
>>
>> + /* SynIC and Vmbus devices require messages/signals hypercalls */
>> + if (hyperv_feat_enabled(cpu, HYPERV_FEAT_SYNIC) &&
>> + !cpu->hyperv_synic_kvm_only) {
>> + c->ebx |= HV_POST_MESSAGES | HV_SIGNAL_EVENTS;
>
> Why exactly is the hyperv_synic_kvm_only check needed?
>
'hyperv_synic_kvm_only' means SynIC is only used for in-KVM stimers and
in this case Post Messages / Signal Events hypercalls are not used. KVM
will also return an error to the guest directly (as it can't forward
them to QEMU). No need to expose HV_POST_MESSAGES | HV_SIGNAL_EVENTS.
> Is the hyperv_synic_kvm_only check the only reason this was done
> here and not at kvm_hyperv_properties?
>
Yes, basically.
>
>> + }
>> +
>> /* Not exposed by KVM but needed to make CPU hotplug in Windows work */
>> c->edx |= HV_CPU_DYNAMIC_PARTITIONING_AVAILABLE;
>>
>> --
>> 2.31.1
>>
--
Vitaly
