Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

Zack Rusin
2024-11-05T04:59:50Z

On Mon, Nov 4, 2024 at 5:13 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On Wed, Oct 30, 2024 at 4:35 AM Zack Rusin <zack.rusin@broadcom.com> wrote:
> >
> > VMware products handle hypercalls in userspace. Give KVM the ability
> > to run VMware guests unmodified by fowarding all hypercalls to the
> > userspace.
> >
> > Enabling of the KVM_CAP_X86_VMWARE_HYPERCALL_ENABLE capability turns
> > the feature on - it's off by default. This allows vmx's built on top
> > of KVM to support VMware specific hypercalls.
>
> Hi Zack,

Hi, Paolo.

Thank you for looking at this.

> is there a spec of the hypercalls that are supported by userspace? I
> would like to understand if there's anything that's best handled in
> the kernel.

There's no spec but we have open headers listing the hypercalls.
There's about a 100 of them (a few were deprecated), the full
list starts here:
https://github.com/vmware/open-vm-tools/blob/739c5a2f4bfd4cdda491e6a6f6869d88c0bd6972/open-vm-tools/lib/include/backdoor_def.h#L97
They're not well documented, but the names are pretty self-explenatory.

> If we allow forwarding _all_ hypercalls to userspace, then people will
> use it for things other than VMware and there goes all hope of
> accelerating stuff in the kernel in the future.
>
> So even having _some_ checks in the kernel before going out to
> userspace would keep that door open, or at least try.

Doug just looked at this and I think I might have an idea on how to
limit the scope at least a bit: if you think it would help we could
limit forwarding of hypercalls to userspace only to those that that
come with a BDOOR_MAGIC (which is 0x564D5868) in eax. Would that help?

> Patch 1 instead looks good from an API point of view.

Ah, great, thanks!

z