RE: [RFC PATCH v9 2/5] iommu/vt-d: break out ATS Invalidation if target device is gone

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Ethan Zhao <haifeng.zhao@linux.intel.com>,
	"bhelgaas@google.com" <bhelgaas@google.com>,
	"baolu.lu@linux.intel.com" <baolu.lu@linux.intel.com>,
	"dwmw2@infradead.org" <dwmw2@infradead.org>,
	"will@kernel.org" <will@kernel.org>,
	"robin.murphy@arm.com" <robin.murphy@arm.com>,
	"lukas@wunner.de" <lukas@wunner.de>
Cc: "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [RFC PATCH v9 2/5] iommu/vt-d: break out ATS Invalidation if target device is gone
Date: Fri, 29 Dec 2023 08:06:15 +0000	[thread overview]
Message-ID: <BN9PR11MB5276ED0949E04BD25A4F91428C9DA@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <17725ef5-d777-420b-9586-4aade103282e@linux.intel.com>

> From: Ethan Zhao <haifeng.zhao@linux.intel.com>
> Sent: Thursday, December 28, 2023 9:03 PM
> 
> On 12/28/2023 4:30 PM, Tian, Kevin wrote:
> >> From: Ethan Zhao <haifeng.zhao@linux.intel.com>
> >> Sent: Thursday, December 28, 2023 8:17 AM
> >>
> >> For those endpoint devices connect to system via hotplug capable ports,
> >> users could request a warm reset to the device by flapping device's link
> >> through setting the slot's link control register, as pciehp_ist() DLLSC
> >> interrupt sequence response, pciehp will unload the device driver and
> >> then power it off. thus cause an IOMMU device-TLB invalidation (Intel
> >> VT-d spec, or ATS Invalidation in PCIe spec r6.1) request for device to
> >> be sent and a long time completion/timeout waiting in interrupt context.
> > is above describing the behavior of safe removal or surprise removal?
> 
> bring the link down is a kind of surprise removal for hotplug capable
> 
> device.

then it's better to make it clear from beginning that this is about surprise
removal in which device is removed and cannot respond to on-going
ATS invalidation request incurred in the removal process.

safe removal should be immune from this problem as the device is still
responsive in the whole removal process.

> >> [ 4223.822628] Call Trace:
> >> [ 4223.822628]  qi_flush_dev_iotlb+0xb1/0xd0
> >> [ 4223.822628]  __dmar_remove_one_dev_info+0x224/0x250
> >> [ 4223.822629]  dmar_remove_one_dev_info+0x3e/0x50
> >> [ 4223.822629]  intel_iommu_release_device+0x1f/0x30
> >> [ 4223.822629]  iommu_release_device+0x33/0x60
> >> [ 4223.822629]  iommu_bus_notifier+0x7f/0x90
> >> [ 4223.822630]  blocking_notifier_call_chain+0x60/0x90
> >> [ 4223.822630]  device_del+0x2e5/0x420
> >> [ 4223.822630]  pci_remove_bus_device+0x70/0x110
> >> [ 4223.822630]  pciehp_unconfigure_device+0x7c/0x130

I'm curious why this doesn't occur earlier when the device is
detached from the driver. At that point presumably the device
should be detached from the DMA domain which involves
ATS invalidation too.

> >>
> >>   	while (qi->desc_status[wait_index] != QI_DONE) {
> >> +		/*
> >> +		 * if the device-TLB invalidation target device is gone, don't
> >> +		 * wait anymore, it might take up to 1min+50%, causes
> >> system
> >> +		 * hang. (see Implementation Note in PCIe spec r6.1 sec
> >> 10.3.1)
> >> +		 */
> >> +		if ((type == QI_DIOTLB_TYPE || type == QI_DEIOTLB_TYPE)
> >> && pdev)
> >> +			if (!pci_device_is_present(pdev))
> >> +				break;
> > I'm not sure it's the right thing to do. Such check should be put in the
> > caller which has the device pointer and can already know it's absent
> > to not call those cache invalidation helpers.
> 
> Here is to handle such case, the invalidation request is sent, but the
> 
> device is just pulled out at that moment.
> 

one problem - the caller could pass multiple descriptors while type
only refers to the 1st descriptor.

btw is it an Intel specific problem? A quick glance at smmu driver
suggests the same problem too:

  arm_smmu_atc_inv_domain()
    arm_smmu_cmdq_batch_submit()
      arm_smmu_cmdq_issue_cmdlist()
        arm_smmu_cmdq_poll_until_sync()
          __arm_smmu_cmdq_poll_until_consumed()

/*
 * Wait until the SMMU cons index passes llq->prod.
 * Must be called with the cmdq lock held in some capacity.
 */
static int __arm_smmu_cmdq_poll_until_consumed(struct arm_smmu_device *smmu,
                                               struct arm_smmu_ll_queue *llq)

is there a more general way to solve it?