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

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Message-ID: <20231115124221.GE3818@noisy.programming.kicks-ass.net>
Date:   Wed, 15 Nov 2023 13:42:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 Kernel <x86@...nel.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>,
        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 Sat, Nov 11, 2023 at 08:16:39PM -0800, Jacob Pan wrote:

> +static __always_inline inline void handle_pending_pir(struct pi_desc *pid, struct pt_regs *regs)
> +{
> +	int i, vec = FIRST_EXTERNAL_VECTOR;
> +	u64 pir_copy[4];
> +
> +	/*
> +	 * Make a copy of PIR which contains IRQ pending bits for vectors,
> +	 * then invoke IRQ handlers for each pending vector.
> +	 * If any new interrupts were posted while we are processing, will
> +	 * do again before allowing new notifications. The idea is to
> +	 * minimize the number of the expensive notifications if IRQs come
> +	 * in a high frequency burst.
> +	 */
> +	for (i = 0; i < 4; i++)
> +		pir_copy[i] = raw_atomic64_xchg((atomic64_t *)&pid->pir_l[i], 0);

Might as well use arch_xchg() and save the atomic64_t casting.

> +
> +	/*
> +	 * Ideally, we should start from the high order bits set in the PIR
> +	 * since each bit represents a vector. Higher order bit position means
> +	 * the vector has higher priority. But external vectors are allocated
> +	 * based on availability not priority.
> +	 *
> +	 * EOI is included in the IRQ handlers call to apic_ack_irq, which
> +	 * allows higher priority system interrupt to get in between.
> +	 */
> +	for_each_set_bit_from(vec, (unsigned long *)&pir_copy[0], 256)
> +		call_irq_handler(vec, regs);
> +
> +}