[Bug 219602] New: Default of kvm.enable_virt_at_load breaks other virtualization solutions (by default)

[Bug 219602] New: Default of kvm.enable_virt_at_load breaks other virtualization solutions (by default)

[bugzilla-daemon]

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

            Bug ID: 219602
           Summary: Default of kvm.enable_virt_at_load breaks other
                    virtualization solutions (by default)
           Product: Virtualization
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: acmelab@proton.me
        Regression: No

Previously (kernel 6.11 and lower) VMs of other virtualization solutions such
as VirtualBox could be started with the KVM module enabled.

Since 6.12 setting kvm.enable_virt_at_load=0 needs to be set at boot if the KVM
module is enabled, if you want to use other virtualization solution beside KVM. 

The commit that introduced the new behavior is
https://github.com/torvalds/linux/commit/b4886fab6fb620b96ad7eeefb9801c42dfa91741.
To be clear it is not about the feature itself, but the default. I am also
aware that this might be intentional; but given it deviates from
previous/expected behavior, it seems like a regression.

The fix would be to set the feature to be off by default. This would be
trivial, but first I need to know for sure that this is considered a bug.

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[Bug 219602] Default of kvm.enable_virt_at_load breaks other virtualization solutions (by default)

[bugzilla-daemon]

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

Pinky (acmelab@proton.me) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
 Bisected commit-id|                            |b4886fab6fb620b96ad7eeefb98
                   |                            |01c42dfa91741
     Kernel Version|                            |6.12
         Regression|No                          |Yes

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

[bugzilla-daemon]

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

Pinky (acmelab@proton.me) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Default of                  |By default
                   |kvm.enable_virt_at_load     |kvm.enable_virt_at_load
                   |breaks other virtualization |breaks other virtualization
                   |solutions (by default)      |solutions

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Re: [Bug 219602] New: Default of kvm.enable_virt_at_load breaks other virtualization solutions (by default)

[Christoph Hellwig]

On Mon, Dec 16, 2024 at 09:15:22AM +0000, bugzilla-daemon@kernel.org wrote:
> Previously (kernel 6.11 and lower) VMs of other virtualization solutions such
> as VirtualBox could be started with the KVM module enabled.

There is no other in-tree user of the hardware virtualization
capabilities, so this can't break anything by definition.

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

[bugzilla-daemon]

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

--- Comment #1 from hch@infradead.org ---
On Mon, Dec 16, 2024 at 09:15:22AM +0000, bugzilla-daemon@kernel.org wrote:
> Previously (kernel 6.11 and lower) VMs of other virtualization solutions such
> as VirtualBox could be started with the KVM module enabled.

There is no other in-tree user of the hardware virtualization
capabilities, so this can't break anything by definition.

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

[bugzilla-daemon]

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

--- Comment #2 from Pinky (acmelab@proton.me) ---
(In reply to hch from comment #1)
> On Mon, Dec 16, 2024 at 09:15:22AM +0000, bugzilla-daemon@kernel.org wrote:
> > Previously (kernel 6.11 and lower) VMs of other virtualization solutions
> such
> > as VirtualBox could be started with the KVM module enabled.
> 
> There is no other in-tree user of the hardware virtualization
> capabilities, so this can't break anything by definition.

That sounds like a dead-end argument to me. What is the use case for this new
feature actually? What would be the disadvantage to turn this off by default?

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

[bugzilla-daemon]

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

Sean Christopherson (seanjc@google.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 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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[Bug 219602] By default kvm.enable_virt_at_load breaks other virtualization solutions

[bugzilla-daemon]

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

Pinky (acmelab@proton.me) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |ANSWERED

--- Comment #4 from Pinky (acmelab@proton.me) ---
(In reply to Sean Christopherson from comment #3)
> 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

Thanks for the longer explanation (including the link's content). Given that
this was discussed already, I am going to close this then.

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