Re: [PATCH] KVM: arm64: Constrain the host to the maximum shared SVE VL with pKVM

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Sep 10, 2024 at 09:21:17PM +0100, Mark Brown wrote:
> When pKVM saves and restores the host floating point state on a SVE system
> it programs the vector length in ZCR_EL2.LEN to be whatever the maximum VL
> for the PE is but uses a buffer allocated with kvm_host_sve_max_vl, the
> maximum VL shared by all PEs in the system. This means that if we run on a
> system where the maximum VLs are not consistent we will overflow the buffer
> on PEs which support larger VLs.
> 
> Since the host will not currently attempt to make use of non-shared VLs fix
> this by explicitly setting the EL2 VL to be the maximum shared VL when we
> save and restore. This will enforce the limit on host VL usage. Should we
> wish to support asymmetric VLs this code will need to be updated along with
> the required changes for the host, patches have previously been posted:
> 
>   https://lore.kernel.org/r/20240730-kvm-arm64-fix-pkvm-sve-vl-v6-0-cae8a2e0bd66@kernel.org
> 
> Fixes: b5b9955617bc ("KVM: arm64: Eagerly restore host fpsimd/sve state in pKVM")
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 2 +-
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c      | 7 ++++---
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index f59ccfe11ab9..ab1425baf0e9 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -339,7 +339,7 @@ static inline void __hyp_sve_save_host(void)
>  	struct cpu_sve_state *sve_state = *host_data_ptr(sve_state);
>  
>  	sve_state->zcr_el1 = read_sysreg_el1(SYS_ZCR);
> -	write_sysreg_s(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
> +	write_sysreg_s(sve_vq_from_vl(kvm_host_sve_max_vl), SYS_ZCR_EL2);
>  	__sve_save_state(sve_state->sve_regs + sve_ffr_offset(kvm_host_sve_max_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..90ff79950912 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -45,10 +45,11 @@ static void __hyp_sve_restore_host(void)
>  	 * the host. The layout of the data when saving the sve state depends
>  	 * on the VL, so use a consistent (i.e., the maximum) host VL.
>  	 *
> -	 * Setting ZCR_EL2 to ZCR_ELx_LEN_MASK sets the effective length
> -	 * supported by the system (or limited at EL3).
> +	 * Note that this constrains the PE to the maximum shared VL
> +	 * that was discovered, if we wish to use larger VLs this will
> +	 * need to be revisited.
>  	 */
> -	write_sysreg_s(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
> +	write_sysreg_s(sve_vq_from_vl(kvm_host_sve_max_vl), SYS_ZCR_EL2);
>  	__sve_restore_state(sve_state->sve_regs + sve_ffr_offset(kvm_host_sve_max_vl),
>  			    &sve_state->fpsr,
>  			    true);
> 
> ---
> base-commit: 7c626ce4bae1ac14f60076d00eafe71af30450ba
> change-id: 20240910-kvm-arm64-limit-guest-vl-d5fba0c7cc7b
> 
> Best regards,
> -- 

It's a shame that the allocation logic lives somewhere far far away,
so the code here needs to "guess" a consistent correct bounding VL for
the host here.  If there's a change of policy later on, someone may
need to remember to fix both places.

Just a thought, but is there a way of getting the relevant bits of
logic at least into the same file?

Cheers
---Dave