Re: [PATCH 0/2] KVM: arm64: nv: Fix sysreg RESx-ication

[Joey Gouly <joey.gouly@arm.com>]

On Sun, Jan 12, 2025 at 04:50:27PM +0000, Marc Zyngier wrote:
> Joey recently reported that some rather basic tests were failing on
> NV, and managed to track it down to critical register fields (such as
> HCR_EL2.E2H) not having their expect value.
> 
> Further investigation has outlined a couple of critical issues:
> 
> - Evaluating HCR_EL2.E2H must always be done with a sanitising
>   accessor, no ifs, no buts. Given that KVM assumes a fixed value for
>   this bit, we cannot leave it to the guest to mess with.
> 
> - Resetting the sysreg file must result in the RESx bits taking
>   effect. Otherwise, we may end-up making the wrong decision (see
>   above), and we definitely expose invalid values to the guest. Note
>   that because we compute the RESx masks very late in the VM setup, we
>   need to apply these masks at that particular point as well.
> 
> The two patches in this series are enough to fix the current set of
> issues, but __vcpu_sys_reg() needs some extra work as it is doing the
> wrong thing when used as a lvalue. I'll post a separate series for
> that, as the two problems are fairly orthogonal, and this results in a
> significant amount of churn.
> 
> All kudos to Joey for patiently tracking that one down. This was
> hidden behind a myriad of other issues, and nailing this sucker down
> is nothing short of a debugging lesson. Drinks on me next time.
> 
> Unless someone shouts, I'll take this in for 6.14.
> 

Testing using the updated kvm-arm64/nv-next branch:
Tested-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>

Thanks!
Joey

> Marc Zyngier (2):
>   KVM: arm64: nv: Always evaluate HCR_EL2 using sanitising accessors
>   KVM: arm64: nv: Apply RESx settings to sysreg reset values
> 
>  arch/arm64/include/asm/kvm_emulate.h | 36 ++++++++++++----------------
>  arch/arm64/include/asm/kvm_nested.h  |  2 +-
>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c   |  4 ++--
>  arch/arm64/kvm/nested.c              |  9 +++++--
>  arch/arm64/kvm/sys_regs.c            |  5 +++-
>  5 files changed, 29 insertions(+), 27 deletions(-)
> 
> -- 
> 2.39.2
>