linux-kernel - Re: [PATCH v3 11/18] dmaengine: idxd: ims setup for the vdcm

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Message-ID: <87r1q92mkx.fsf@nanos.tec.linutronix.de>
Date:   Thu, 08 Oct 2020 09:39:26 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Jiang <dave.jiang@...el.com>, vkoul@...nel.org,
        megha.dey@...el.com, maz@...nel.org, bhelgaas@...gle.com,
        alex.williamson@...hat.com, jacob.jun.pan@...el.com,
        ashok.raj@...el.com, jgg@...lanox.com, yi.l.liu@...el.com,
        baolu.lu@...el.com, kevin.tian@...el.com, sanjay.k.kumar@...el.com,
        tony.luck@...el.com, jing.lin@...el.com, dan.j.williams@...el.com,
        kwankhede@...dia.com, eric.auger@...hat.com, parav@...lanox.com,
        rafael@...nel.org, netanelg@...lanox.com, shahafs@...lanox.com,
        yan.y.zhao@...ux.intel.com, pbonzini@...hat.com,
        samuel.ortiz@...el.com, mona.hossain@...el.com
Cc:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-pci@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 11/18] dmaengine: idxd: ims setup for the vdcm

On Wed, Oct 07 2020 at 14:54, Dave Jiang wrote:
> On 9/30/2020 12:57 PM, Thomas Gleixner wrote:
>> Aside of that this is fiddling in the IMS storage array behind the irq
>> chips back without any comment here and a big fat comment about the
>> shared usage of ims_slot::ctrl in the irq chip driver.
>> 
> This is to program the pasid fields in the IMS table entry. Was
> thinking the pasid fields may be considered device specific so didn't
> attempt to add the support to the core code.

Well, the problem is that this is not really irq chip functionality.

But the PASID programming needs to touch the IMS storage which is also
touched by the irq chip.

This might be correct as is, but without a big fat comment explaining
WHY it is safe to do so without any form of serialization this is just
voodoo and unreviewable.

Can you please explain when the PASID is programmed and what the state
of the interrupt is at that point? Is this a one off setup operation or
does this happen dynamically at random points during runtime?

This needs to be clarified first.

Thanks,

        tglx