Re: [PATCH] ARM64: errata: Add workaround for HIP10/HIP10C erratum 162200803

[Marc Zyngier <maz@kernel.org>]

On Thu, 26 Jun 2025 13:41:42 +0100,
Zhou Wang <wangzhou1@hisilicon.com> wrote:
> 
> For GICv4.0 of Hip10 and Hip10C, it has a SoC bug with vPE schedule:
> when multiple vPEs are sending vpe schedule/deschedule commands
> concurrently and repeatedly, some vPE schedule command may not be
> scheduled, and it will cause the command timeout.
> 
> The hardware implementation is that there is one GIC hardware in one CPU die,
> which handles all vPE schedule operations one by one in all CPUs of this die.
> The bug is that if the number of queued vPE schedule operations is more
> than a certain value, the last vPE schedule operation will be lost.
> 
> One possible way to solve this problem is to limit the number of vLPIs, so
> the hardware could spend less time to scan virtual pending table when it
> handles the vPE schedule operations, so the queued vPE schedule operations
> will never be more than above certain value.
> 
> Given the number of CPUs of die, and imagine there is 100 vPE schedule
> operations per second one CPU, it can be calculated that we can limit
> the number of vLPI to 4096 for virtual machine to avoid the issue.
> 
> Signed-off-by: Zhou Wang <wangzhou1@hisilicon.com>
> ---
>  Documentation/arch/arm64/silicon-errata.rst |  2 ++
>  arch/arm64/Kconfig                          | 12 ++++++++++++
>  arch/arm64/include/asm/cputype.h            |  4 ++++
>  arch/arm64/kernel/cpu_errata.c              | 15 +++++++++++++++
>  arch/arm64/kvm/vgic/vgic-mmio-v3.c          |  5 +++++
>  arch/arm64/tools/cpucaps                    |  1 +
>  include/linux/irqchip/arm-gic-v3.h          |  1 +
>  7 files changed, 40 insertions(+)
> 

[...]

> diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> index ae4c0593d114..495a56e9dc4b 100644
> --- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> +++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> @@ -81,6 +81,11 @@ static unsigned long vgic_mmio_read_v3_misc(struct kvm_vcpu *vcpu,
>  		if (vgic_has_its(vcpu->kvm)) {
>  			value |= (INTERRUPT_ID_BITS_ITS - 1) << 19;
>  			value |= GICD_TYPER_LPIS;
> +			/* Limit the number of vlpis to 4096 */
> +			if (cpus_have_final_cap(ARM64_WORKAROUND_HISI_162200803) &&
> +			    kvm_vgic_global_state.has_gicv4 &&
> +			    !kvm_vgic_global_state.has_gicv4_1)
> +				value |= 11 << GICD_TYPER_NUM_LPIS_SHIFT;

This really doesn't solve your problem. Yes, the guest *may* honor
this limit. But KVM doesn't care and will happily allocate 2^16 vLPIs
if the guest asks -- there is no code enforcing this limit.

And even if we did. What would we do on a MAPTI command that tries to
map a vLPI outside of the allowed range? Do we need to tell the guest
it has screwed up?

Thanks,

	M.

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