linux-kernel - Re: [PATCH 07/17] vfio/pci: Preserve per-interrupt contexts

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Message-ID: <20240205153509.333c2c95.alex.williamson@redhat.com>
Date: Mon, 5 Feb 2024 15:35:09 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: jgg@...dia.com, yishaih@...dia.com,
 shameerali.kolothum.thodi@...wei.com, kevin.tian@...el.com,
 kvm@...r.kernel.org, dave.jiang@...el.com, ashok.raj@...el.com,
 linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH 07/17] vfio/pci: Preserve per-interrupt contexts

On Thu,  1 Feb 2024 20:57:01 -0800
Reinette Chatre <reinette.chatre@...el.com> wrote:

> MSI and MSI-X interrupt management for PCI passthrough devices create
> a new per-interrupt context every time an interrupt is allocated,
> freeing it when the interrupt is freed.
> 
> The per-interrupt context contains the properties of a particular
> interrupt. Without a property that persists across interrupt allocation
> and free it is acceptable to always create a new per-interrupt context.
> 
> INTx interrupt context has a "masked" property that persists across
> allocation and free and thus preserves its interrupt context
> across interrupt allocation and free calls.
> 
> MSI and MSI-X interrupts already remain allocated across interrupt
> allocation and free requests, additionally maintaining the
> individual interrupt context is a reflection of this existing
> behavior and matches INTx behavior so that more code can be shared.
> 
> An additional benefit is that maintaining interrupt context supports
> a potential future use case of emulated interrupts, where the
> "is this interrupt emulated" is a property that needs to persist
> across allocation and free requests.
> 
> Persistent interrupt contexts means that existence of per-interrupt
> context no longer implies a valid trigger, pointers to freed memory
> should be cleared, and a new per-interrupt context cannot be assumed
> needing allocation when an interrupt is allocated.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
> Note to maintainers:
> This addition originally formed part of the IMS work below that mostly
> ignored INTx. This work focuses on INTx, MSI, MSI-X where this addition
> is relevant.
> https://lore.kernel.org/lkml/cover.1696609476.git.reinette.chatre@intel.com
> 
>  drivers/vfio/pci/vfio_pci_intrs.c | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index 31f73c70fcd2..7ca2b983b66e 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -427,7 +427,7 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  
>  	ctx = vfio_irq_ctx_get(vdev, vector);
>  
> -	if (ctx) {
> +	if (ctx && ctx->trigger) {
>  		irq_bypass_unregister_producer(&ctx->producer);
>  		irq = pci_irq_vector(pdev, vector);
>  		cmd = vfio_pci_memory_lock_and_enable(vdev);
> @@ -435,8 +435,9 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  		vfio_pci_memory_unlock_and_restore(vdev, cmd);
>  		/* Interrupt stays allocated, will be freed at MSI-X disable. */
>  		kfree(ctx->name);
> +		ctx->name = NULL;

Setting ctx->name = NULL is not strictly necessary and does not match
the INTx code that we're claiming to try to emulate.  ctx->name is only
tested immediately after allocation below, otherwise it can be inferred
from ctx->trigger.  Thanks,

Alex

>  		eventfd_ctx_put(ctx->trigger);
> -		vfio_irq_ctx_free(vdev, ctx, vector);
> +		ctx->trigger = NULL;
>  	}
>  
>  	if (fd < 0)
> @@ -449,16 +450,17 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  			return irq;
>  	}
>  
> -	ctx = vfio_irq_ctx_alloc(vdev, vector);
> -	if (!ctx)
> -		return -ENOMEM;
> +	/* Per-interrupt context remain allocated. */
> +	if (!ctx) {
> +		ctx = vfio_irq_ctx_alloc(vdev, vector);
> +		if (!ctx)
> +			return -ENOMEM;
> +	}
>  
>  	ctx->name = kasprintf(GFP_KERNEL_ACCOUNT, "vfio-msi%s[%d](%s)",
>  			      msix ? "x" : "", vector, pci_name(pdev));
> -	if (!ctx->name) {
> -		ret = -ENOMEM;
> -		goto out_free_ctx;
> -	}
> +	if (!ctx->name)
> +		return -ENOMEM;
>  
>  	trigger = eventfd_ctx_fdget(fd);
>  	if (IS_ERR(trigger)) {
> @@ -502,8 +504,7 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  	eventfd_ctx_put(trigger);
>  out_free_name:
>  	kfree(ctx->name);
> -out_free_ctx:
> -	vfio_irq_ctx_free(vdev, ctx, vector);
> +	ctx->name = NULL;
>  	return ret;
>  }
>  
> @@ -539,6 +540,7 @@ static void vfio_msi_disable(struct vfio_pci_core_device *vdev,
>  		vfio_virqfd_disable(&ctx->unmask);
>  		vfio_virqfd_disable(&ctx->mask);
>  		vfio_msi_set_vector_signal(vdev, i, -1, index);
> +		vfio_irq_ctx_free(vdev, ctx, i);
>  	}
>  
>  	cmd = vfio_pci_memory_lock_and_enable(vdev);
> @@ -694,7 +696,7 @@ static int vfio_pci_set_msi_trigger(struct vfio_pci_core_device *vdev,
>  
>  	for (i = start; i < start + count; i++) {
>  		ctx = vfio_irq_ctx_get(vdev, i);
> -		if (!ctx)
> +		if (!ctx || !ctx->trigger)
>  			continue;
>  		if (flags & VFIO_IRQ_SET_DATA_NONE) {
>  			eventfd_signal(ctx->trigger);