linux-kernel - Re: [PATCH RFC 03/13] x86: Reserved a per CPU IDT vector for posted MSIs

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Date:   Sat, 9 Dec 2023 13:53:50 -0800
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 Kernel <x86@...nel.org>,
        iommu@...ts.linux.dev, 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>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, maz@...nel.org,
        peterz@...radead.org, seanjc@...gle.com,
        Robin Murphy <robin.murphy@....com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH RFC 03/13] x86: Reserved a per CPU IDT vector for posted
 MSIs

Hi Thomas,

On Wed, 06 Dec 2023 17:47:07 +0100, Thomas Gleixner <tglx@...utronix.de>
wrote:

> On Sat, Nov 11 2023 at 20:16, Jacob Pan wrote:
> 
> $Subject: x86/vector: Reserve ...
> 
> > Under posted MSIs, all device MSIs are multiplexed into a single CPU  
> 
> Under?
Will change to "When posted MSIs are enabled, "
 
> > notification vector. MSI handlers will be de-multiplexed at run-time by
> > system software without IDT delivery.
> >
> > This vector has a priority class below the rest of the system vectors.  
> 
> Why?
I was thinking system interrupt can preempt device posted MSIs. But if
nested interrupt is not an option, there is no need.

> > Potentially, external vector number space for MSIs can be expanded to
> > the entire 0-256 range.  
> 
> Don't even mention this. It's wishful thinking and has absolutely
> nothing to do with the patch at hand.
will remove.

> > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > ---
> >  arch/x86/include/asm/irq_vectors.h | 15 ++++++++++++++-
> >  1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/irq_vectors.h
> > b/arch/x86/include/asm/irq_vectors.h index 3a19904c2db6..077ca38f5a91
> > 100644 --- a/arch/x86/include/asm/irq_vectors.h
> > +++ b/arch/x86/include/asm/irq_vectors.h
> > @@ -99,9 +99,22 @@
> >  
> >  #define LOCAL_TIMER_VECTOR		0xec
> >  
> > +/*
> > + * Posted interrupt notification vector for all device MSIs delivered
> > to
> > + * the host kernel.
> > + *
> > + * Choose lower priority class bit [7:4] than other system vectors such
> > + * that it can be preempted by the system interrupts.  
> 
> That's future music and I'm not convinced at all that we want to allow
> nested interrupts with all their implications. Stack depth is the least
> of the worries here. There are enough other assumptions about interrupts
> not nesting in Linux.
> 
Then, should we allow limited interrupt priority inversion while processing
posted MSIs?

In the current code, without preemption, we effectively already allow one
low priority to block higher ones.

I don't know the other worries caused by nested interrupts, still
experimenting/studying, but here I am thinking it is just one-deep nesting.
Posted MSI notifications are not allowed to nest, so does other system
interrupts.

> > + * It is also higher than all external vectors but it should not matter
> > + * in that external vectors for posted MSIs are in a different number
> > space.  
> 
> This whole priority muck is pointless. The kernel never used it and will
> never use it.
OK. Perhaps I didn't make it clear, I am just trying to let system
interrupt, such as timer, to preempt posted MSI. Not TPR/PPR etc.

> > + */
> > +#define POSTED_MSI_NOTIFICATION_VECTOR	0xdf  
> 
> So this just wants to go into the regular system vector number space
> until there is a conclusion whether we can and want to allow nested
> interrupts. Premature optimization is just creating more confusion than
> value.
Make sense, for this patchset I didn't include the preemption patch since I
am not sure yet.
I should use the next system vector.


Thanks,

Jacob