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

[Eduardo Habkost]

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?

> 
> 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?

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
> 

-- 
Eduardo