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

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Message-ID: <CABQX2QMR=Nsn23zojFdhemR7tvGUz6_UM8Rgf6WLsxwDqoFtxg@mail.gmail.com>
Date: Mon, 4 Nov 2024 23:59:39 -0500
From: Zack Rusin <zack.rusin@...adcom.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, Doug Covelli <doug.covelli@...adcom.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>, Shuah Khan <shuah@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...hat.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Joel Stanley <joel@....id.au>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

On Mon, Nov 4, 2024 at 5:13 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On Wed, Oct 30, 2024 at 4:35 AM Zack Rusin <zack.rusin@...adcom.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