linux-kernel - Re: [PATCH v2 11/13] iommu/vt-d: Make posted MSI an opt-in cmdline option

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Date: Sat, 6 Apr 2024 12:31:14 +0800
From: Robert Hoo <robert.hoo.linux@...il.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 LKML <linux-kernel@...r.kernel.org>, X86 Kernel <x86@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, iommu@...ts.linux.dev,
 Thomas Gleixner <tglx@...utronix.de>, Lu Baolu <baolu.lu@...ux.intel.com>,
 kvm@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
 Joerg Roedel <joro@...tes.org>, "H. Peter Anvin" <hpa@...or.com>,
 Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>
Cc: Paul Luse <paul.e.luse@...el.com>, Dan Williams
 <dan.j.williams@...el.com>, Jens Axboe <axboe@...nel.dk>,
 Raj Ashok <ashok.raj@...el.com>, "Tian, Kevin" <kevin.tian@...el.com>,
 maz@...nel.org, seanjc@...gle.com, Robin Murphy <robin.murphy@....com>,
 jim.harris@...sung.com, a.manzanares@...sung.com,
 Bjorn Helgaas <helgaas@...nel.org>, guang.zeng@...el.com
Subject: Re: [PATCH v2 11/13] iommu/vt-d: Make posted MSI an opt-in cmdline
 option

On 4/6/2024 6:31 AM, Jacob Pan wrote:
> Add a command line opt-in option for posted MSI if CONFIG_X86_POSTED_MSI=y.
> 
> Also introduce a helper function for testing if posted MSI is supported on
> the platform.
> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
>   Documentation/admin-guide/kernel-parameters.txt |  1 +
>   arch/x86/include/asm/irq_remapping.h            | 11 +++++++++++
>   drivers/iommu/irq_remapping.c                   | 13 ++++++++++++-
>   3 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index bb884c14b2f6..e5fd02423c4c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2251,6 +2251,7 @@
>   			no_x2apic_optout
>   				BIOS x2APIC opt-out request will be ignored
>   			nopost	disable Interrupt Posting
> +			posted_msi enable MSIs delivered as posted interrupts
>   
>   	iomem=		Disable strict checking of access to MMIO memory
>   		strict	regions from userspace.
> diff --git a/arch/x86/include/asm/irq_remapping.h b/arch/x86/include/asm/irq_remapping.h
> index 7a2ed154a5e1..e46bde61029b 100644
> --- a/arch/x86/include/asm/irq_remapping.h
> +++ b/arch/x86/include/asm/irq_remapping.h
> @@ -50,6 +50,17 @@ static inline struct irq_domain *arch_get_ir_parent_domain(void)
>   	return x86_vector_domain;
>   }
>   
> +#ifdef CONFIG_X86_POSTED_MSI
> +extern int enable_posted_msi;
> +
> +static inline bool posted_msi_supported(void)
> +{
> +	return enable_posted_msi && irq_remapping_cap(IRQ_POSTING_CAP);
> +}

Out of this patch set's scope, but, dropping into irq_remappping_cap(), I'd like 
to bring this change for discussion:

diff --git a/drivers/iommu/irq_remapping.c b/drivers/iommu/irq_remapping.c
index 4047ac396728..ef2de9034897 100644
--- a/drivers/iommu/irq_remapping.c
+++ b/drivers/iommu/irq_remapping.c
@@ -98,7 +98,7 @@ void set_irq_remapping_broken(void)

  bool irq_remapping_cap(enum irq_remap_cap cap)
  {
-       if (!remap_ops || disable_irq_post)
+       if (!remap_ops || disable_irq_remap)
                 return false;

         return (remap_ops->capability & (1 << cap));

1. irq_remapping_cap() is to exam some cap, though at present it has only 1 cap, 
i.e. IRQ_POSTING_CAP, simply return false just because of disable_irq_post isn't 
good. Instead, IRQ_REMAP is the foundation of all remapping caps.
2. disable_irq_post is used by Intel iommu code only, here irq_remapping_cap() 
is common code. e.g. AMD iommu code doesn't use it to judge set cap of irq_post 
or not.

> +#else
> +static inline bool posted_msi_supported(void) { return false; };
> +#endif