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

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Message-ID: <CABQX2QMznYZiVm40Ligq+pFKmEkVpScW+zcKYbPpGgm0=S2Xkg@mail.gmail.com>
Date: Wed, 23 Apr 2025 07:43:38 -0400
From: Zack Rusin <zack.rusin@...adcom.com>
To: Xin Li <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, Doug Covelli <doug.covelli@...adcom.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, 
	Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 4/5] KVM: x86: Add support for legacy VMware backdoors
 in nested setups

On Wed, Apr 23, 2025 at 3:56 AM Xin Li <xin@...or.com> wrote:
>
> 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@...adcom.com>
> > Cc: Doug Covelli <doug.covelli@...adcom.com>
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Jonathan Corbet <corbet@....net>
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: x86@...nel.org
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Zack Rusin <zack.rusin@...adcom.com>
> > Cc: kvm@...r.kernel.org
> > Cc: linux-doc@...r.kernel.org
> > Cc: linux-kernel@...r.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?

It was rebased on top of v6.14.

> > 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.

Are you sure about that? This code is already used elsewhere in the
file  (for KVM_CAP_EXIT_ON_EMULATION_FAILURE) so, ignoring the fact
that it's arguable whether IS_ZERO_OR_ONE is more readable than & ~1,
if we use a macro for the vmware caps and not for
KVM_CAP_EXIT_ON_EMULATION_FAILURE then the code would be inconsistent
and that decreases the readability.

Or are you saying that since I'm already there you'd like to see a
completely separate patch that defines some kind of IS_ZERO_OR_ONE
macro for KVM, use it for KVM_CAP_EXIT_ON_EMULATION_FAILURE and, once
that lands then I can make use of it in this series?

z

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