Re: [RFC v2 0/5] ARM Nested Virt Support

[Marc Zyngier <maz@kernel.org>]

On 2024-02-09 18:57, Peter Maydell wrote:
> On Fri, 9 Feb 2024 at 16:00, Eric Auger <eric.auger@redhat.com> wrote:
>> 
>> This series adds ARM Nested Virtualization support in KVM mode.
>> This is a respin of previous contributions from Miguel [1] and Haibo 
>> [2].
>> 
>> This was tested with Marc's v11 [3] on Ampere HW with fedora L1 guest 
>> and
>> L2 guests booted without EDK2. However it does not work yet with
>> EDK2 but it looks unrelated to this qemu integration (host hard 
>> lockups).
>> 
>> The host needs to be booted with "kvm-arm.mode=nested" option and
>> qemu needs to be invoked with :
>> 
>> -machine virt,virtualization=on
>> 
>> There is a known issue with hosts supporting SVE. Kernel does not 
>> support both
>> SVE and NV2 and the current qemu integration has an issue with the
>> scratch_host_vcpu startup because both are enabled if exposed by the 
>> kernel.
>> This is independent on whether sve is disabled on the command line. 
>> Unfortunately
>> I lost access to the HW that expose that issue so I couldn't fix it in 
>> this
>> version.
> 
> You can probably repro that by running the whole setup under
> QEMU's FEAT_NV emulation, which will be able to give you a CPU
> with both FEAT_NV and SVE.
> 
> Personally I think that this is a kernel missing-feature that
> should really be fixed as part of getting the kernel patches
> upstreamed. There's no cause to force every userspace VMM to
> develop extra complications for this.

I don't plan to make NV visible to userspace before this is fixed.
Which may delay KVM NV by another year or five, but I don't think
anyone is really waiting for it anyway.

         M.
-- 
Jazz is not dead. It just smells funny...