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To:     Dexuan Cui <decui@microsoft.com>,
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CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>
Subject: RE: [PATCH v4 6/6] x86/hyperv: Fix serial console interrupts for TDX
 guests
Thread-Topic: [PATCH v4 6/6] x86/hyperv: Fix serial console interrupts for TDX
 guests
Thread-Index: AQHZalujP8O9vIzAEkmR/lI5q4nag68mWtKA
Date:   Tue, 11 Apr 2023 17:13:49 +0000
Message-ID: <BYAPR21MB1688798EA7DD5ED309BAE0F6D79A9@BYAPR21MB1688.namprd21.prod.outlook.com>
References: <20230408204759.14902-1-decui@microsoft.com>
 <20230408204759.14902-7-decui@microsoft.com>
In-Reply-To: <20230408204759.14902-7-decui@microsoft.com>
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From: Dexuan Cui <decui@microsoft.com> Sent: Saturday, April 8, 2023 1:48 P=
M
>=20
> When a 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().
>=20
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>=20
> Changes since v1:
>     None.
>=20
>  arch/x86/kernel/cpu/mshyperv.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>=20
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyper=
v.c
> index e9106c9d92f81..deedced0f2bb0 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -318,6 +318,26 @@ static void __init hv_smp_prepare_cpus(unsigned int =
max_cpus)
>  }
>  #endif
>=20
> +/*
> + * When a TDX guest runs on Hyper-V, the firmware sets the HW_REDUCED fl=
ag: see
> + * acpi_tb_create_local_fadt(). Consequently ttyS0 interrupts can't work=
 because
> + * request_irq() -> ... -> irq_to_desc() returns NULL for ttyS0. This ha=
ppens
> + * 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_irq=
s() ->
> + * find_irq_entry() fails to find the legacy irqs from the array, and he=
nce
> + * doesn't create the necessary irq description info.
> + *
> + * Copy arch/x86/kernel/acpi/boot.c: acpi_generic_reduced_hw_init() but =
doesn't
> + * change 'legacy_pic', so it keeps its default value 'default_legacy_pi=
c' in
> + * mp_config_acpi_legacy_irqs(), which sees a non-zero nr_legacy_irqs(),=
 and
> + * eventually serial console interrupts can work properly.

I had a little trouble parsing this comment.  Slightly better wording is:

    * Clone arch/x86/kernel/acpi/boot.c: acpi_generic_reduced_hw_init() her=
e,
    * except don't change 'legacy_pic'.  It keeps its default value 'defaul=
t_legacy_pic'.
    * mp_config_acpi_legacy_irqs() sees a non-zero nr_legacy_irqs(), and
    * eventually serial console interrupts can work properly.

Otherwise,

Reviewed-by: Michael Kelley <mikelley@microsoft.com>

> + */
> +static void __init reduced_hw_init(void)
> +{
> +	x86_init.timers.timer_init	=3D x86_init_noop;
> +	x86_init.irqs.pre_vector_init	=3D x86_init_noop;
> +}
> +
>  static void __init ms_hyperv_init_platform(void)
>  {
>  	int hv_max_functions_eax;
> @@ -425,6 +445,8 @@ static void __init ms_hyperv_init_platform(void)
>=20
>  			/* A TDX VM must use x2APIC and doesn't use lazy EOI */
>  			ms_hyperv.hints &=3D ~HV_X64_APIC_ACCESS_RECOMMENDED;
> +
> +			x86_init.acpi.reduced_hw_early_init =3D reduced_hw_init;
>  		}
>  	}
>=20
> --
> 2.25.1