[Bug 219602] By default kvm.enable_virt_at_load breaks other virtualization solutions

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=219602

Sean Christopherson (seanjc@google.com) changed:

           What    |Removed                     |Added
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                 CC|                            |seanjc@google.com

--- Comment #3 from Sean Christopherson (seanjc@google.com) ---
From the kernel docs:

        kvm.enable_virt_at_load=[KVM,ARM64,LOONGARCH,MIPS,RISCV,X86]
                        If enabled, KVM will enable virtualization in hardware
                        when KVM is loaded, and disable virtualization when KVM
                        is unloaded (if KVM is built as a module).

                        If disabled, KVM will dynamically enable and disable
                        virtualization on-demand when creating and destroying
                        VMs, i.e. on the 0=>1 and 1=>0 transitions of the
                        number of VMs.

                        Enabling virtualization at module lode avoids potential
                        latency for creation of the 0=>1 VM, as KVM serializes
                        virtualization enabling across all online CPUs.  The
                        "cost" of enabling virtualization when KVM is loaded,
                        is that doing so may interfere with using out-of-tree
                        hypervisors that want to "own" virtualization hardware.

In addition to the latency angle, TDX support is effectively going to require
VMX to be enabled when KVM is loaded, i.e. trying to do something different
will only delay the inevitable.

FWIW, Paolo and I do want to get to a state where out-of-tree hypervisors don't
need to do weird things, but it'll take some time to get to that state.

https://lore.kernel.org/all/ZwQjUSOle6sWARsr@google.com

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