Re: [RFC 0/4] kvm: arm64: emulate ID registers

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On Thu, Aug 13, 2020 at 02:05:13PM +0800, Peng Liang wrote:
> In AArch64, guest will read the same values of the ID regsiters with
> host.  Both of them read the values from arm64_ftr_regs.  This patch
> series add support to emulate and configure ID registers so that we can
> control the value of ID registers that guest read.
> 
> Peng Liang (4):
>   arm64: add a helper function to traverse arm64_ftr_regs
>   kvm: arm64: emulate the ID registers
>   kvm: arm64: make ID registers configurable
>   kvm: arm64: add KVM_CAP_ARM_CPU_FEATURE extension
> 
>  arch/arm64/include/asm/cpufeature.h |  2 ++
>  arch/arm64/include/asm/kvm_host.h   |  2 ++
>  arch/arm64/kernel/cpufeature.c      | 13 ++++++++
>  arch/arm64/kvm/arm.c                | 21 ++++++++++++
>  arch/arm64/kvm/sys_regs.c           | 50 ++++++++++++++++++++++-------
>  include/uapi/linux/kvm.h            | 12 +++++++
>  6 files changed, 89 insertions(+), 11 deletions(-)
> 
> -- 
> 2.18.4
> 

Hi Peng,

After having looked at the QEMU series I don't believe this is the right
approach to CPU models. As stated in my reply [*] this approach appears
to just be shifting the problem wholesale to the user. On the KVM side,
I think we should start by auditing all the ID registers to see what
should be masked and what should be under user control. Then, we need
to consider what MIDR a guest that can migrate between hosts of different
MIDRs should have. And that'll require addressing the errata problem in
one way or another.

[*] https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg02587.html

Thanks,
drew

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