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

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Date: Mon, 8 Apr 2024 16:33:12 -0700
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: Robert Hoo <robert.hoo.linux@...il.com>
Cc: 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>, 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,
 jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v2 11/13] iommu/vt-d: Make posted MSI an opt-in cmdline
 option

Hi Robert,

On Sat, 6 Apr 2024 12:31:14 +0800, Robert Hoo <robert.hoo.linux@...il.com>
wrote:

> 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.
I agree, posting should be treated as a sub-capability of remapping.
IRQ_POSTING_CAP is only set when remapping is on.

We need to delete this such that posting is always off when remapping is
off.

--- a/drivers/iommu/intel/irq_remapping.c
+++ b/drivers/iommu/intel/irq_remapping.c
@@ -1038,11 +1038,7 @@ static void disable_irq_remapping(void)
                iommu_disable_irq_remapping(iommu);
        }
 
-       /*
-        * Clear Posted-Interrupts capability.
-        */
-       if (!disable_irq_post)
-               intel_irq_remap_ops.capability &= ~(1 << IRQ_POSTING_CAP);
+       intel_irq_remap_ops.capability &= ~(1 << IRQ_POSTING_CAP);
 } 

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


Thanks,

Jacob