Re: [RFC PATCH 00/14] Support multiple KVM modules on the same host

Hou Wenlong
2026-01-05T07:48:24Z

On Tue, Nov 28, 2023 at 10:10:20AM -0800, Sean Christopherson wrote:
> On Fri, Nov 17, 2023, Lai Jiangshan wrote:
> > On Wed, Nov 8, 2023 at 4:20 AM Anish Ghulati <aghulati@google.com> wrote:
> > >
> > > This series is a rough, PoC-quality RFC to allow (un)loading and running
> > > multiple KVM modules simultaneously on a single host, e.g. to deploy
> > > fixes, mitigations, and/or new features without having to drain all VMs
> > > from the host. Multi-KVM will also allow running the "same" KVM module
> > > with different params, e.g. to run trusted VMs with different mitigations.
> > >
> > > The goal of this RFC is to get feedback on the idea itself and the
> > > high-level approach. In particular, we're looking for input on:
> > >
> > > - Combining kvm_intel.ko and kvm_amd.ko into kvm.ko
> > > - Exposing multiple /dev/kvmX devices via Kconfig
> > > - The name and prefix of the new base module
> > >
> > > Feedback on individual patches is also welcome, but please keep in mind
> > > that this is very much a work in-progress
> >
> > Hello Anish
> >
> > Scarce effort on multi-KVM can be seen in the mail list albeit many
> > companies enable multi-KVM internally.
> >
> > I'm glad that you took a big step in upstreaming it. And I hope it
> > can be materialized soon.
> >
> >
> > >
> > > - Move system-wide virtualization resource management to a new base
> > > module to avoid collisions between different KVM modules, e.g. VPIDs
> > > and ASIDs need to be unique per VM, and callbacks from IRQ handlers need
> > > to be mediated so that things like PMIs get to the right KVM instance.
> >
> > perf_register_guest_info_callbacks() also accesses to system-wide resources,
> > but I don't see its relating code including kvm_guest_cbs being moved to AVC.
>
> Yeah, that's on the TODO list. IIRC, the plan is to have VAC register a single
> callback with perf, and then have VAC deal with invoking the callback(s) for the
> correct KVM instance.
>
> > > - Refactor KVM to make all upgradable assets visible only to KVM, i.e.
> > > make KVM a black box, so that the layout/size of things like "struct
> > > kvm_vcpu" isn't exposed to the kernel at-large.
> > >
> > > - Fold kvm_intel.ko and kvm_amd.ko into kvm.ko to avoid complications
> > > having to generate unique symbols for every symbol exported by kvm.ko.
> >
> > The sizes of kvm_intel.ko and kvm_amd.ko are big, and there
> > is only 1G in the kernel available for modules. So I don't think folding
> > two vendors' code into kvm.ko is a good idea.
> >
> > Since the symbols in the new module are invisible outside, I recommend:
> > new kvm_intel.ko = kvm_intel.ko + kvm.ko
> > new kvm_amd.ko = kvm_amd.ko + kvm.ko
>
> Yeah, Paolo also suggested this at LPC.
>
> > > - Add a Kconfig string to allow defining a device and module postfix at
> > > build time, e.g. to create kvmX.ko and /dev/kvmX.
> > >
> > > The proposed name of the new base module is vac.ko, a.k.a.
> > > Virtualization Acceleration Code (Unupgradable Units Module). Childish
> > > humor aside, "vac" is a unique name in the kernel and hopefully in x86
> > > and hardware terminology, is a unique name in the kernel and hopefully
> > > in x86 and hardware terminology, e.g. `git grep vac_` yields no hits in
> > > the kernel. It also has the same number of characters as "kvm", e.g.
> > > the namespace can be modified without needing whitespace adjustment if
> > > we want to go that route.
> >
> > How about the name kvm_base.ko?
> >
> > And the variable/function name in it can still be kvm_foo (other than
> > kvm_base_foo).
>
> My preference is to have a unique name that allows us to differentitate between
> the "base" module/code and KVM code. Verbal conversations about all of this get
> quite confusing because it's not always clear whether "base KVM" refers to what
> is currently kvm.ko, or what would become kvm_base.ko/vac.ko.
>
Hi, Sean and Anish.

Sorry for revisiting this topic after a long time. I haven't seen any
new updates regarding this topic/series, and I didn’t find any recent
activity on the GitHub repository. Is the multi-KVM topic still being
considered for upstreaming, or is there anything blocking this?

As Lai pointed out, we also have a similar multi-KVM implementation in
our internal environment, so we are quite interested in this topic.
Recently, when we upgraded our kernel version, we found that maintaining
multi-KVM has become a significant burden. We are willing to move
forward with it if multi-KVM is still accepted for upstream. So I look
forward to feedback from the maintainers.

From what I've seen, the recent patch set that enables VMX/SVM during
booting is a good starting point for multi-KVM as well.

Thanks!