Re: [RFC PATCH RESEND 0/4] hw/arm/virt: Add support for Target Implementation CPUs

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Aug 01 2025, Shameer Kolothum <shameerkolothum@gmail.com> wrote:

> From: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
>
> (This is my third attempt to send this series to qemu-devel / qemu-arm.
> Previous attempts failed with the following errors:
>
> qemu-arm@nongnu.org: connect to eggs.gnu.org[209.51.188.92]:25: Connection timed out  
> qemu-devel@nongnu.org: connect to eggs.gnu.org[209.51.188.92]:25: Connection timed out  
>
> I'm not sure if the issue is related to my setup, so I'm now trying to send the
> patches via my Gmail account.
>
> Apologies if you’ve already received this series individually, and thank you
> for your patience.)

FWIW, all of your mails have made it into my inbox -- let's see if at
least my reply shows up on the lists :/

>
> Hi,
>
> On ARM64 platforms, many errata workarounds are based on CPU identification
> registers, particularly MIDR_EL1 and REVIDR_EL1. Some of these errata
> require corresponding fixes in the guest kernel as well. This becomes a
> challenge during guest migration between hosts with different MIDR/REVIDR
> values, even when the VMM provides a common baseline feature set using
> the recently introduced writable ID register support.
>
> To address this, the Linux kernel now provides:
>
>  -A mechanism to expose implementation CPU data to guests via KVM
>   hypercalls [0]:
>
>   ARM_SMCCC_VENDOR_HYP_KVM_DISCOVER_IMPL_VER_FUNC_ID
>   ARM_SMCCC_VENDOR_HYP_KVM_DISCOVER_IMPL_CPUS_FUNC_ID
>
>  -A way to override implementation ID registers (MIDR, REVIDR, AIDR),
>   which were previously treated as invariant [1].
>
> What this RFC does:
>
> This RFC enables the virt machine to specify Target Implementation CPUs
> via QEMU command-line, exposing them to the guest via KVM hypercalls.
>
> Example use case (hosts with differing MIDRs):
>
> ./qemu-system-aarch64 -machine virt,gic-version=3 \
>   -enable-kvm \
>   -bios QEMU_EFI.fd \
>   -m 1G -smp cpus=4,maxcpus=4 \
>   -cpu host \
>   -M impl-cpu.0.midr=0x481fd010,impl-cpu.0.revidr=0,impl-cpu.0.aidr=0x551,\
>    impl-cpu.1.midr=0x481fd011,impl-cpu.1.revidr=0,impl-cpu.1.aidr=0x551 \
>   -kernel Image-6.16-rc2 \
>   -initrd rootfs-iperf.cpio \
>   -append "rdinit=init console=ttyAMA0 root=/dev/vda rw earlycon=pl011,0x9000000 kpti=off" \
>   -net none \
>   -nographic
>
> Highlights and Notes:
>
>  -This is based on Connie’s series[2] “kvm/arm: Introduce a customizable
>   aarch64 KVM host model”.

Hm, I *really* need to do a respin of that one, at least to bring it up
to date with the code that changed underneath...

>   
>  -Currently, no validation is performed on user-specified implementation CPU
>   data. It’s assumed that one of the entries reflects the actual host CPU.
>   If needed, a check can be added to enforce this. All other target CPU
>   definitions are treated as user-driven with no internal verification.

Should that check rather be done by management software on top? Although
I assume that this is the part that in practice will come up with the
list of possible target cpus.

>
>  -For now, only the DISCOVER_IMPL_* hypercalls are supported on the QEMU side.
>   See patches 1 and 3 for implementation details. These are loosely inspired
>   by Salil's vCPU hotplug series patch here [3]. This may require a more
>   generic approach if we have to support other SMCCC hyper calls.
>
>  -This series uses macros and definitions from the kernel's
>   include/linux/arm-smccc.h. I am not sure whether these should be duplicated 
>   in QEMU or moved to a UAPI header in the kernel. For now I am carrying
>   it in the test branch here,
>   https://github.com/hisilicon/qemu/commit/8f583c76a2a5f60cf0f9616d61d0863e524ef388
>
> This is lightly tested on HiSilicon ARM platforms. A complete test branch
> can be found here,
> https://github.com/hisilicon/qemu/tree/arm-cpu-model-rfcv3-tgt-impl-cpu-rfcv1
>
> Please take a look and let me know.
>
> Thanks,
> Shameer
>
> [0] https://lore.kernel.org/all/20250221140229.12588-1-shameerali.kolothum.thodi@huawei.com/
> [1] https://lore.kernel.org/all/20250225005401.679536-1-oliver.upton@linux.dev/
> [2] https://lore.kernel.org/qemu-devel/20250414163849.321857-1-cohuck@redhat.com/
> [3] https://lore.kernel.org/qemu-devel/20240614001510.202991-1-salil.mehta@huawei.com/
>
> Shameer Kolothum (4):
>   target/arm/kvm: Introduce helper to check target impl CPU support
>   target/arm/kvm: Add QAPI struct ArmTargetImplCPU
>   target/arm/kvm: Handle KVM Target Imp CPU hypercalls
>   hw/arm/virt: Add Target Implementation CPU support
>
>  hw/arm/virt.c         |  66 +++++++++++++++
>  include/hw/arm/virt.h |   3 +
>  qapi/machine.json     |  34 ++++++++
>  target/arm/kvm.c      | 187 ++++++++++++++++++++++++++++++++++++++++++
>  target/arm/kvm_arm.h  |  20 +++++
>  5 files changed, 310 insertions(+)
>
> -- 
> 2.34.1