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

Lai Jiangshan
2023-11-17T08:54:14Z

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.

>
> - 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

>
> - 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).

Thanks
Lai