linux-kernel - Re: [PATCH RFC 09/13] x86/irq: Install posted MSI notification handler

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Date:   Wed, 06 Dec 2023 20:50:24 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
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,
        seanjc@...gle.com, Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH RFC 09/13] x86/irq: Install posted MSI notification handler

On Wed, Nov 15 2023 at 13:56, Peter Zijlstra wrote:
>
> Would it not make more sense to write things something like:
>
> bool handle_pending_pir()
> {
> 	bool handled = false;
> 	u64 pir_copy[4];
>
> 	for (i = 0; i < 4; i++) {
> 		if (!pid-pir_l[i]) {
> 			pir_copy[i] = 0;
> 			continue;
> 		}
>
> 		pir_copy[i] = arch_xchg(&pir->pir_l[i], 0);
> 		handled |= true;
> 	}
>
> 	if (!handled)
> 		return handled;
>
> 	for_each_set_bit()
> 		....
>
> 	return handled.
> }

I don't understand what the whole copy business is about. It's
absolutely not required.

static bool handle_pending_pir(unsigned long *pir)
{
        unsigned int idx, vec;
	bool handled = false;
        unsigned long pend;
        
        for (idx = 0; offs < 4; idx++) {
                if (!pir[idx])
                	continue;
		pend = arch_xchg(pir + idx, 0);
                for_each_set_bit(vec, &pend, 64)
			call_irq_handler(vec + idx * 64, NULL);
                handled = true;
	}
        return handled;
}

No?

> sysvec_posted_blah_blah()
> {
> 	bool done = false;
> 	bool handled;
>
> 	for (;;) {
> 		handled = handle_pending_pir();
> 		if (done)
> 			break;
> 		if (!handled || ++loops > MAX_LOOPS) {

That does one loop too many. Should be ++loops == MAX_LOOPS. No?

> 			pi_clear_on(pid);
> 			/* once more after clear_on */
> 			done = true;
> 		}
> 	}
> }
>
>
> Hmm?

I think that can be done less convoluted.

{
	struct pi_desc *pid = this_cpu_ptr(&posted_interrupt_desc);
	struct pt_regs *old_regs = set_irq_regs(regs);
        int loops;

	for (loops = 0;;) {
        	bool handled = handle_pending_pir((unsigned long)pid->pir);

                if (++loops > MAX_LOOPS)
                	break;

                if (!handled || loops == MAX_LOOPS) {
                	pi_clear_on(pid);
                        /* Break the loop after handle_pending_pir()! */
                        loops = MAX_LOOPS;
                }
	}

	...
	set_irq_regs(old_regs);
}

Hmm? :)