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

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Message-ID: <13e03561-eff6-a90f-902f-05252e1b8949@gmail.com>
Date:   Wed, 23 Aug 2023 16:17:54 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Dexuan Cui <decui@...rosoft.com>, ak@...ux.intel.com,
        arnd@...db.de, bp@...en8.de, brijesh.singh@....com,
        dan.j.williams@...el.com, dave.hansen@...el.com,
        dave.hansen@...ux.intel.com, haiyangz@...rosoft.com, hpa@...or.com,
        jane.chu@...cle.com, kirill.shutemov@...ux.intel.com,
        kys@...rosoft.com, linux-hyperv@...r.kernel.org, luto@...nel.org,
        mingo@...hat.com, peterz@...radead.org, rostedt@...dmis.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        tglx@...utronix.de, tony.luck@...el.com, wei.liu@...nel.org,
        Jason@...c4.com, nik.borisov@...e.com, mikelley@...rosoft.com
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, Tianyu.Lan@...rosoft.com,
        rick.p.edgecombe@...el.com, andavis@...hat.com, mheslin@...hat.com,
        vkuznets@...hat.com, xiaoyao.li@...el.com
Subject: Re: [PATCH 4/9] x86/hyperv: Fix serial console interrupts for fully
 enlightened TDX guests

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@...rosoft.com>
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>

Reviewed-by: Tianyu Lan <tiala@...rosoft.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;
>   			}
>   		}
>   	}