Re: [PATCH 4/9] x86/hyperv: Fix serial console interrupts for fully enlightened TDX guests

[Tianyu Lan <ltykernel@gmail.com>]

On 8/12/2023 6:18 AM, Dexuan Cui wrote:
> When a fully enlightened TDX guest runs on Hyper-V, the UEFI firmware sets
> the HW_REDUCED flag and consequently ttyS0 interrupts can't work. Fix the
> issue by overriding x86_init.acpi.reduced_hw_early_init().
> 
> Reviewed-by: Michael Kelley <mikelley@microsoft.com>
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

Reviewed-by: Tianyu Lan <tiala@microsoft.com>
> ---
>   arch/x86/kernel/cpu/mshyperv.c | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 507df0f64ae18..b4214e37e9124 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -324,6 +324,26 @@ static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
>   }
>   #endif
>   
> +/*
> + * When a fully enlightened TDX VM runs on Hyper-V, the firmware sets the
> + * HW_REDUCED flag: refer to acpi_tb_create_local_fadt(). Consequently ttyS0
> + * interrupts can't work because request_irq() -> ... -> irq_to_desc() returns
> + * NULL for ttyS0. This happens because mp_config_acpi_legacy_irqs() sees a
> + * nr_legacy_irqs() of 0, so it doesn't initialize the array 'mp_irqs[]', and
> + * later setup_IO_APIC_irqs() -> find_irq_entry() fails to find the legacy irqs
> + * from the array and hence doesn't create the necessary irq description info.
> + *
> + * Clone arch/x86/kernel/acpi/boot.c: acpi_generic_reduced_hw_init() here,
> + * except don't change 'legacy_pic', which keeps its default value
> + * 'default_legacy_pic'. This way, mp_config_acpi_legacy_irqs() sees a non-zero
> + * nr_legacy_irqs() and eventually serial console interrupts works properly.
> + */
> +static void __init reduced_hw_init(void)
> +{
> +	x86_init.timers.timer_init	= x86_init_noop;
> +	x86_init.irqs.pre_vector_init	= x86_init_noop;
> +}
> +
>   static void __init ms_hyperv_init_platform(void)
>   {
>   	int hv_max_functions_eax;
> @@ -442,6 +462,8 @@ static void __init ms_hyperv_init_platform(void)
>   
>   				/* Don't trust Hyper-V's TLB-flushing hypercalls. */
>   				ms_hyperv.hints &= ~HV_X64_REMOTE_TLB_FLUSH_RECOMMENDED;
> +
> +				x86_init.acpi.reduced_hw_early_init = reduced_hw_init;
>   			}
>   		}
>   	}