linux-kernel - Re: [PATCH v6 15/20] vfio/mdev: idxd: ims domain setup for the vdcm

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Message-ID: <87mtsb3sth.ffs@nanos.tec.linutronix.de>
Date:   Mon, 31 May 2021 16:02:02 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>, Dave Jiang <dave.jiang@...el.com>
Cc:     alex.williamson@...hat.com, kwankhede@...dia.com, vkoul@...nel.org,
        megha.dey@...el.com, jacob.jun.pan@...el.com, ashok.raj@...el.com,
        yi.l.liu@...el.com, baolu.lu@...el.com, kevin.tian@...el.com,
        sanjay.k.kumar@...el.com, tony.luck@...el.com,
        dan.j.williams@...el.com, eric.auger@...hat.com,
        pbonzini@...hat.com, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v6 15/20] vfio/mdev: idxd: ims domain setup for the vdcm

On Sun, May 23 2021 at 20:50, Jason Gunthorpe wrote:
> On Fri, May 21, 2021 at 05:20:37PM -0700, Dave Jiang wrote:
>> @@ -77,8 +80,18 @@ int idxd_mdev_host_init(struct idxd_device *idxd, struct mdev_driver *drv)
>>  		return rc;
>>  	}
>>  
>> +	ims_info.max_slots = idxd->ims_size;
>> +	ims_info.slots = idxd->reg_base + idxd->ims_offset;
>> +	idxd->ims_domain = pci_ims_array_create_msi_irq_domain(idxd->pdev, &ims_info);
>> +	if (!idxd->ims_domain) {
>> +		dev_warn(dev, "Fail to acquire IMS domain\n");
>> +		iommu_dev_disable_feature(dev, IOMMU_DEV_FEAT_AUX);
>> +		return -ENODEV;
>> +	}
>
> I'm quite surprised that every mdev doesn't create its own ims_domain
> in its probe function.

What for?

> This places a global total limit on the # of vectors which makes me
> ask what was the point of using IMS in the first place ?

That depends on how IMS is implemented. The IDXD variant has a fixed
sized message store which is shared between all subdevices, so yet
another domain would not provide any value.

For the case where the IMS store is seperate, you still have one central
irqdomain per physical device. The domain allocation function can then
create storage on demand or reuse existing storage and just fill in the
pointers.

Thanks,

        tglx