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

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Date: Wed, 5 Jun 2024 13:12:34 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Mark Brown <broonie@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	James Morse <james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH 3/4] KVM: arm64: Fix FFR offset calculation for pKVM host
 state save and restore

Hi Mark,

On Wed, Jun 5, 2024 at 12:50 PM Mark Brown <broonie@...nel.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.
>
> Fixes: b5b9955617bc ("KVM: arm64: Eagerly restore host fpsimd/sve state in pKVM")
> Signed-off-by: Mark Brown <broonie@...nel.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);
>
>  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);
>  }

If my understanding of the spec is correct (which more often than not
it isn't), I don't think we have an issue as long as we use the same
value in the offset on saving/restoring, and that that value
represents the maximum possible value.

On the other hand, if my understanding is wrong, then we might need to
also fix __efi_fpsimd_begin()/__efi_fpsimd_end() in
arch/arm64/kernel/fpsimd.c, as well as vcpu_sve_pffr() in
arch/arm64/include/asm/kvm_host.h

What do you think?
/fuad


> 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);
>
> --
> 2.39.2
>