Re: [PATCH v6 1/5] KVM: arm64: Share all userspace hardened thread data with the hypervisor

Marc Zyngier
2024-03-31T10:00:48Z

On Fri, 29 Mar 2024 00:13:42 +0000,
Mark Brown <broonie@kernel.org> wrote:
>
> As part of the lazy FPSIMD state transitioning done by the hypervisor we
> currently share the userpsace FPSIMD state in thread->uw.fpsimd_state with
> the host. Since this struct is non-extensible userspace ABI we have to keep

Using the same representation is just pure convenience, and nothing
requires us to use the it in the kernel/hypervisor.

> the definition as is but the addition of FPMR in the 2023 dpISA means that
> we will want to share more storage with the host. To facilitate this
> refactor the current code to share the entire thread->uw rather than just
> the one field.

So this increase the required sharing with EL2 from 528 bytes to
560. Not a huge deal, but definitely moving in the wrong direction. Is
there any plans to add more stuff to this structure that wouldn't be
*directly* relevant to the hypervisor?

>
> The large number of references to fpsimd_state make it very inconvenient
> to add an additional wrapper struct.
>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
> arch/arm64/include/asm/kvm_host.h | 3 ++-
> arch/arm64/include/asm/processor.h | 2 +-
> arch/arm64/kvm/fpsimd.c | 13 ++++++-------
> arch/arm64/kvm/hyp/include/hyp/switch.h | 2 +-
> arch/arm64/kvm/hyp/nvhe/hyp-main.c | 4 ++--
> 5 files changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 9e8a496fb284..8a251f0da900 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -27,6 +27,7 @@
> #include <asm/fpsimd.h>
> #include <asm/kvm.h>
> #include <asm/kvm_asm.h>
> +#include <asm/processor.h>
> #include <asm/vncr_mapping.h>
>
> #define __KVM_HAVE_ARCH_INTC_INITIALIZED
> @@ -640,7 +641,7 @@ struct kvm_vcpu_arch {
> struct kvm_guest_debug_arch vcpu_debug_state;
> struct kvm_guest_debug_arch external_debug_state;
>
> - struct user_fpsimd_state *host_fpsimd_state; /* hyp VA */
> + struct thread_struct_uw *host_uw; /* hyp VA */
> struct task_struct *parent_task;

Well, this is going away, and you know it.

M.

--
Without deviation from the norm, progress is not possible.