linux-kernel - Re: [PATCH,RFC] iommu/vt-d: Convert dmar

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Date:   Sat, 29 Oct 2022 16:12:04 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Lennert Buytenhek <buytenh@...tstofly.org>
Cc:     baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Scarlett Gourley <scarlett@...sta.com>,
        James Sewart <jamessewart@...sta.com>,
        Jack O'Sullivan <jack@...sta.com>
Subject: Re: [PATCH,RFC] iommu/vt-d: Convert dmar_fault IRQ to a threaded IRQ

On 2022/10/27 16:19, Lennert Buytenhek wrote:
>>> iommu/amd already uses a threaded IRQ handler for its I/O page fault
>>> reporting, and so it already has this advantage.
>>>
>>> When IRQ remapping is enabled, iommu/vt-d will try to set up its
>>> dmar_fault IRQ handler from start_kernel() -> x86_late_time_init()
>>> -> apic_intr_mode_init() -> apic_bsp_setup() ->
>>> irq_remap_enable_fault_handling() -> enable_drhd_fault_handling(),
>>> which happens before kthreadd is started, and trying to set up a
>>> threaded IRQ handler this early on will oops.  However, there
>>> doesn't seem to be a reason why iommu/vt-d needs to set up its fault
>>> reporting IRQ handler this early, and if we remove the IRQ setup code
>>> from enable_drhd_fault_handling(), the IRQ will be registered instead
>>> from pci_iommu_init() -> intel_iommu_init() -> init_dmars(), which
>>> seems to work just fine.
>> At present, we cannot do so. Because the VT-d interrupt remapping and
>> DMA remapping can be independently enabled. In another words, it's a
>> possible case where interrupt remapping is enabled while DMA remapping
>> is not.
> Is there a way I can test this easily?
> 
> I think we should be able to handle the "interrupt remapping enabled
> but DMA remapping disabled" case in the same way, by registering the
> dmar_fault IRQ sometime after kthreadd has been started.  I don't think
> the dmar_fault handler performs any function that is critical for the
> operation of the IOMMU, and I think that we can defer setting it up
> until whenever is convenient.

Another possible way is not to split VT-d DMA remapping and interrupt
remapping. The possible case of "intr remapping enabled but DMA
remapping not" that I can imagine is that the guest VM doesn't want DMA
translation because of poor efficiency. If so, the overhead impacted by
DMA translation can be eliminated through "iommu=pt" or kernel build
configuration. Of course, there may also be some special needs that I
did not think of.

Best regards,
baolu