Re: [PATCH 03/12] KVM: arm64: Force SRE traps when SRE access is not enabled

[Oliver Upton <oliver.upton@linux.dev>]

Hey,

On Tue, Aug 20, 2024 at 11:03:40AM +0100, Marc Zyngier wrote:
> We so far only write the ICH_HCR_EL2 config in two situations:
> 
> - when we need to emulate the GICv3 CPU interface due to HW bugs
> 
> - when we do direct injection, as the virtual CPU interface needs
>   to be enabled
> 
> This is all good. But it also means that we don't do anything special
> when we emulate a GICv2, or that there is no GIC at all.
> 
> What happens in this case when the guest uses the GICv3 system
> registers? The *guest* gets a trap for a sysreg access (EC=0x18)
> while we'd really like it to get an UNDEF.
> 
> Fixing this is a bit involved:
> 
> - we need to set all the required trap bits (TC, TALL0, TALL1, TDIR)
> 
> - for these traps to take effect, we need to (counter-intuitively)
>   set ICC_SRE_EL1.SRE to 1 so that the above traps take priority.
> 
> Note that doesn't fully work when GICv2 emulation is enabled, as
> we cannot set ICC_SRE_EL1.SRE to 1 (it breaks Group0 delivery as
> IRQ).

Just to make sure I'm following completely, GICv2-on-GICv3 guest sees
the (barf) architected behavior of sysreg traps going to EL1.

> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/hyp/vgic-v3-sr.c | 22 ++++++++++++++++------
>  arch/arm64/kvm/vgic/vgic-v3.c   |  5 ++++-
>  2 files changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/vgic-v3-sr.c b/arch/arm64/kvm/hyp/vgic-v3-sr.c
> index 7b397fad26f2..a184def8f5ad 100644
> --- a/arch/arm64/kvm/hyp/vgic-v3-sr.c
> +++ b/arch/arm64/kvm/hyp/vgic-v3-sr.c
> @@ -268,8 +268,16 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
>  	 * starting to mess with the rest of the GIC, and VMCR_EL2 in
>  	 * particular.  This logic must be called before
>  	 * __vgic_v3_restore_state().
> +	 *
> +	 * However, if the vgic is disabled (ICH_HCR_EL2.EN==0), no GIC is
> +	 * provisionned at all. In order to prevent illegal accesses to the

typo: provisioned

> +	 * system registers to trap to EL1 (duh), force ICC_SRE_EL1.SRE to 1
> +	 * so that the trap bits can take effect. Yes, we *loves* the GIC.
>  	 */
> -	if (!cpu_if->vgic_sre) {
> +	if (!(cpu_if->vgic_hcr & ICH_HCR_EN)) {
> +		write_gicreg(ICC_SRE_EL1_SRE, ICC_SRE_EL1);
> +		isb();
> +	} else if (!cpu_if->vgic_sre) {
>  		write_gicreg(0, ICC_SRE_EL1);
>  		isb();
>  		write_gicreg(cpu_if->vgic_vmcr, ICH_VMCR_EL2);
> @@ -288,8 +296,9 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
>  	}
>  
>  	/*
> -	 * Prevent the guest from touching the GIC system registers if
> -	 * SRE isn't enabled for GICv3 emulation.
> +	 * Prevent the guest from touching the ICC_SRE_EL1 system
> +	 * register. Note that this may not have any effect, as
> +	 * ICC_SRE_EL2.Enable being RAO/WI is a valid implementation.

So this behavior is weird but still 'safe' as, ICC_SRE_EL1.SRE is also RAO/WI
and the HCR traps are still effective. Right?

>  	 */
>  	write_gicreg(read_gicreg(ICC_SRE_EL2) & ~ICC_SRE_EL2_ENABLE,
>  		     ICC_SRE_EL2);
> @@ -297,10 +306,11 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
>  	/*
>  	 * If we need to trap system registers, we must write
>  	 * ICH_HCR_EL2 anyway, even if no interrupts are being
> -	 * injected,
> +	 * injected. Note that this also applies if we don't expect
> +	 * any system register access (GICv2 or no vgic at all).

We don't expect the traps to come in the GICv2 case, though, right?

Looks alright to me otherwise, but blech!

-- 
Thanks,
Oliver