linux-kernel - Re: [PATCH V5 00/11] vfio/pci: Support dynamic allocation of MSI-X interrupts

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Message-ID: <ZGTj8oD4VW15eo6K@nvidia.com>
Date:   Wed, 17 May 2023 11:25:54 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     yishaih@...dia.com, shameerali.kolothum.thodi@...wei.com,
        kevin.tian@...el.com, alex.williamson@...hat.com,
        tglx@...utronix.de, darwi@...utronix.de, kvm@...r.kernel.org,
        dave.jiang@...el.com, jing2.liu@...el.com, ashok.raj@...el.com,
        fenghua.yu@...el.com, tom.zanussi@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 00/11] vfio/pci: Support dynamic allocation of MSI-X
 interrupts

On Thu, May 11, 2023 at 08:44:27AM -0700, Reinette Chatre wrote:
> 
> Qemu allocates interrupts incrementally at the time the guest unmasks an
> interrupt, for example each time a Linux guest runs request_irq().
> 
> Dynamic allocation of MSI-X interrupts was not possible until v6.2 [1].
> This prompted Qemu to, when allocating a new interrupt, first release all
> previously allocated interrupts (including disable of MSI-X) followed
> by re-allocation of all interrupts that includes the new interrupt.
> Please see [2] for a detailed discussion about this issue.
> 
> Releasing and re-allocating interrupts may be acceptable if all
> interrupts are unmasked during device initialization. If unmasking of
> interrupts occur during runtime this may result in lost interrupts.
> For example, consider an accelerator device with multiple work queues,
> each work queue having a dedicated interrupt. A work queue can be
> enabled at any time with its associated interrupt unmasked while other
> work queues are already active. Having all interrupts released and MSI-X
> disabled to enable the new work queue will impact active work queues.
> 
> This series builds on the recent interrupt sub-system core changes
> that added support for dynamic MSI-X allocation after initial MSI-X
> enabling.
> 
> Add support for dynamic MSI-X allocation to vfio-pci. A flag
> indicating lack of support for dynamic allocation already exist:
> VFIO_IRQ_INFO_NORESIZE and has always been set for MSI and MSI-X. With
> support for dynamic MSI-X the flag is cleared for MSI-X when supported,
> enabling Qemu to modify its behavior.
> 
> Any feedback is appreciated

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason