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

From: Marc Zyngier <maz@kernel.org>
To: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Jonathan Corbet <corbet@lwn.net>, Shuah Khan <shuah@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, Dave Martin <Dave.Martin@arm.com>,
	kvmarm@lists.linux.dev, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v6 1/5] KVM: arm64: Share all userspace hardened thread data with the hypervisor
Date: Sun, 31 Mar 2024 11:00:41 +0100	[thread overview]
Message-ID: <87msqesoty.wl-maz@kernel.org> (raw)
In-Reply-To: <20240329-arm64-2023-dpisa-v6-1-ba42db6c27f3@kernel.org>

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.