Re: [PATCH v4 3/4] KVM: arm64: Fix FFR offset calculation for pKVM host state save and restore

[Marc Zyngier <maz@kernel.org>]

On Thu, 04 Jul 2024 18:28:18 +0100,
Mark Brown <broonie@kernel.org> wrote:
> 
> When saving and restoring the SVE state for the host we configure the
> hardware for the maximum VL it supports, but when calculating offsets in
> memory we use the maximum usable VL for the host. Since these two values
> may not be the same this may result in data corruption.  We can just
> read the current VL from the hardware with an instruction so do that
> instead of a saved value, we need to correct the value and this makes
> the consistency obvious.

Which value are we correcting?

> 
> Fixes: b5b9955617bc ("KVM: arm64: Eagerly restore host fpsimd/sve state in pKVM")
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_hyp.h        | 1 +
>  arch/arm64/kvm/hyp/fpsimd.S             | 5 +++++
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 2 +-
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c      | 2 +-
>  4 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
> index b05bceca3385..7510383d78a6 100644
> --- a/arch/arm64/include/asm/kvm_hyp.h
> +++ b/arch/arm64/include/asm/kvm_hyp.h
> @@ -113,6 +113,7 @@ void __fpsimd_save_state(struct user_fpsimd_state *fp_regs);
>  void __fpsimd_restore_state(struct user_fpsimd_state *fp_regs);
>  void __sve_save_state(void *sve_pffr, u32 *fpsr, int save_ffr);
>  void __sve_restore_state(void *sve_pffr, u32 *fpsr, int restore_ffr);
> +int __sve_get_vl(void);

Is there any case where you'd consider returning a signed value? Given
the multiplier of '1', I don't see it likely to happen.

>  
>  u64 __guest_enter(struct kvm_vcpu *vcpu);
>  
> diff --git a/arch/arm64/kvm/hyp/fpsimd.S b/arch/arm64/kvm/hyp/fpsimd.S
> index e950875e31ce..d272dbf36da8 100644
> --- a/arch/arm64/kvm/hyp/fpsimd.S
> +++ b/arch/arm64/kvm/hyp/fpsimd.S
> @@ -31,3 +31,8 @@ SYM_FUNC_START(__sve_save_state)
>  	sve_save 0, x1, x2, 3
>  	ret
>  SYM_FUNC_END(__sve_save_state)
> +
> +SYM_FUNC_START(__sve_get_vl)
> +	_sve_rdvl	0, 1
> +	ret
> +SYM_FUNC_END(__sve_get_vl)
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 0c4de44534b7..06efcca765cc 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -327,7 +327,7 @@ static inline void __hyp_sve_save_host(void)
>  
>  	sve_state->zcr_el1 = read_sysreg_el1(SYS_ZCR);
>  	write_sysreg_s(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
> -	__sve_save_state(sve_state->sve_regs + sve_ffr_offset(kvm_host_sve_max_vl),
> +	__sve_save_state(sve_state->sve_regs + sve_ffr_offset(__sve_get_vl()),
>  			 &sve_state->fpsr,
>  			 true);
>  }
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index f43d845f3c4e..bd8f671e848c 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -49,7 +49,7 @@ static void __hyp_sve_restore_host(void)
>  	 * supported by the system (or limited at EL3).
>  	 */
>  	write_sysreg_s(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
> -	__sve_restore_state(sve_state->sve_regs + sve_ffr_offset(kvm_host_sve_max_vl),
> +	__sve_restore_state(sve_state->sve_regs + sve_ffr_offset(__sve_get_vl()),
>  			    &sve_state->fpsr,
>  			    true);
>  	write_sysreg_el1(sve_state->zcr_el1, SYS_ZCR);
> 

Thanks,

	M.

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