Re: [PATCH v2 4/5] KVM: x86: Add support for legacy VMware backdoors in nested setups

[Xin Li <xin@zytor.com>]

On 4/22/2025 9:12 AM, Zack Rusin wrote:
> Allow handling VMware backdoors by the L0 monitor. This is required on
> setups running Windows VBS, where the L1 will be running Hyper-V which
> can't handle VMware backdoors. Thus on Windows VBS legacy VMware backdoor
> calls issued by the userspace will end up in Hyper-V (L1) and endup
> throwing an error.
> Add a KVM cap that, in nested setups, allows the legacy VMware backdoor
> to be handled by the L0 monitor. Thanks to this we can make sure that
> VMware backdoor is always handled by the correct monitor.
> 
> Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
> Cc: Doug Covelli <doug.covelli@broadcom.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: x86@kernel.org
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Zack Rusin <zack.rusin@broadcom.com>
> Cc: kvm@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>   Documentation/virt/kvm/api.rst  | 14 +++++++++++
>   arch/x86/include/asm/kvm_host.h |  1 +
>   arch/x86/kvm/Kconfig            |  1 +
>   arch/x86/kvm/kvm_vmware.h       | 42 +++++++++++++++++++++++++++++++++
>   arch/x86/kvm/svm/nested.c       |  6 +++++
>   arch/x86/kvm/svm/svm.c          |  3 ++-
>   arch/x86/kvm/vmx/nested.c       |  6 +++++
>   arch/x86/kvm/x86.c              |  8 +++++++
>   include/uapi/linux/kvm.h        |  1 +
>   9 files changed, 81 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 6d3d2a509848..55bd464ebf95 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -8322,6 +8322,20 @@ userspace handling of hypercalls is discouraged. To implement
>   such functionality, use KVM_EXIT_IO (x86) or KVM_EXIT_MMIO
>   (all except s390).
>   
> +7.39 KVM_CAP_X86_VMWARE_NESTED_BACKDOOR_L0
> +------------------------------------------
> +
> +:Architectures: x86
> +:Parameters: args[0] whether the feature should be enabled or not
> +:Returns: 0 on success.
> +
> +Capability allows VMware backdoors to be handled by L0 when running
> +on nested configurations. This is required when, for example
> +running Windows guest with Hyper-V VBS enabled - in that configuration
> +the VMware backdoor calls issued by VMware tools would endup in Hyper-V
> +(L1) which doesn't handle VMware backdoor. Enable this option to have
> +VMware backdoor sent to L0 monitor.
> +
>   8. Other capabilities.
>   ======================
>   

You're not basing the patch set on v6.15-rcX?

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 300cef9a37e2..5dc57bc57851 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4653,6 +4653,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   #ifdef CONFIG_KVM_VMWARE
>   	case KVM_CAP_X86_VMWARE_BACKDOOR:
>   	case KVM_CAP_X86_VMWARE_HYPERCALL:
> +	case KVM_CAP_X86_VMWARE_NESTED_BACKDOOR_L0:
>   #endif
>   		r = 1;
>   		break;
> @@ -6754,6 +6755,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>   		kvm->arch.vmware.hypercall_enabled = cap->args[0];
>   		r = 0;
>   		break;
> +	case KVM_CAP_X86_VMWARE_NESTED_BACKDOOR_L0:
> +		r = -EINVAL;
> +		if (cap->args[0] & ~1)

Replace ~1 with a macro for better readability please.