Re: [PATCH] KVM: arm64: Make L1Ip feature in CTR_EL0 writable from userspace

[Marc Zyngier <maz@kernel.org>]

On Thu, 17 Oct 2024 09:59:25 +0100,
Shameer Kolothum <shameerali.kolothum.thodi@huawei.com> wrote:
> 
> Only allow userspace to set VIPT(0b10) or PIPT(0b11) for L1Ip based on
> what hardware reports as both AIVIVT (0b01) and VPIPT (0b00) are
> documented as reserved.
> 
> Using a VIPT for Guest where hardware reports PIPT may lead to over
> invalidation, but is still correct. Hence, we can allow downgrading
> PIPT to VIPT, but not the other way around.
> 
> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> ---
> This is based on the dicsussion here[0].
> https://lore.kernel.org/kvmarm/0db19a081d9e41f08b0043baeef16f16@huawei.com/
> 
> Also depends on Joey's series[1] as it make use of the ID_FILTERED macro.
> 
> I am not sure we need to explicitly make the ftr type as FTR_LOWER_SAFE
> in kvm_arm64_ftr_safe_value() or as mentioned below can depend on
> arm64_ftr_safe_value() for this ftr bits.

I think relying on the arch code for this is the right thing to
do. This was designed to cope with heterogeneous systems where you
could have both PIPT and VIPT caches in the system, and we don't allow
a late comer to be VIPT if we're decided on PIPT (hence the
FTR_EXACT).

> 
> Please take a look and let me know.
> 
> Thanks,
> Shameer
> 
> [0] https://lore.kernel.org/kvmarm/0db19a081d9e41f08b0043baeef16f16@huawei.com/
> [1] https://lore.kernel.org/kvmarm/20241015133923.3910916-1-joey.gouly@arm.com/
> ---
>  arch/arm64/kvm/sys_regs.c | 32 ++++++++++++++++++++++++++++----
>  1 file changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index d97ccf1c1558..819dcb63febd 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1872,6 +1872,28 @@ static int set_id_aa64pfr1_el1(struct kvm_vcpu *vcpu,
>  	return set_id_reg(vcpu, rd, user_val);
>  }
>  
> +static int set_ctr_el0(struct kvm_vcpu *vcpu,
> +		       const struct sys_reg_desc *rd, u64 user_val)
> +{
> +	u8 user_L1Ip = SYS_FIELD_GET(CTR_EL0, L1Ip, user_val);
> +
> +	/*
> +	 * Both AIVIVT (0b01) and VPIPT (0b00) are documented as reserved.
> +	 * Hence only allow to set VIPT(0b10) or PIPT(0b11) for L1Ip based
> +	 * on what hardware reports.
> +	 *
> +	 * Using a VIPT software model on PIPT will lead to over invalidation,
> +	 * but still correct. Hence, we can allow downgrading PIPT to VIPT,
> +	 * but not the other way around. This is handled via arm64_ftr_safe_value()
> +	 * as CTR_EL0 ftr_bits has L1Ip field type FTR_EXACT with safe value
> +	 * set as VIPT)
> +	 */
> +	if (user_L1Ip < CTR_EL0_L1Ip_VIPT)
> +		return -EINVAL;

I'm not overly fond of this, because the ordering of cache types is
arbitrary (it really is an enumeration). I would rather see the
allowed cache types explicitly listed. It doesn't change a thing, but
makes it all much more readable.

With this fixed:

Reviewed-by: Marc Zyngier <maz@kernel.org>

Thanks,

	M.

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