linux-kernel - Re: [PATCH RFCv2 06/13] iommufd: Make attach

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Message-ID: <84f9ab89-483f-4659-8e7c-5b69748d83b8@intel.com>
Date: Fri, 24 Jan 2025 21:31:59 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>
CC: <jgg@...dia.com>, <will@...nel.org>, <robin.murphy@....com>,
	<kevin.tian@...el.com>, <tglx@...utronix.de>, <maz@...nel.org>,
	<alex.williamson@...hat.com>, <joro@...tes.org>, <shuah@...nel.org>,
	<reinette.chatre@...el.com>, <eric.auger@...hat.com>, <yebin10@...wei.com>,
	<apatel@...tanamicro.com>, <shivamurthy.shastri@...utronix.de>,
	<bhelgaas@...gle.com>, <anna-maria@...utronix.de>, <yury.norov@...il.com>,
	<nipun.gupta@....com>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<kvm@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<patches@...ts.linux.dev>, <jean-philippe@...aro.org>, <mdf@...nel.org>,
	<mshavit@...gle.com>, <shameerali.kolothum.thodi@...wei.com>,
	<smostafa@...gle.com>, <ddutile@...hat.com>
Subject: Re: [PATCH RFCv2 06/13] iommufd: Make attach_handle generic

On 2025/1/20 13:54, Nicolin Chen wrote:
> On Sun, Jan 19, 2025 at 06:40:57PM +0800, Yi Liu wrote:
>> On 2025/1/19 04:32, Nicolin Chen wrote:
>>> On Sat, Jan 18, 2025 at 04:23:22PM +0800, Yi Liu wrote:
>>>> On 2025/1/11 11:32, Nicolin Chen wrote:
>>>>> +static int iommufd_hwpt_attach_device(struct iommufd_hw_pagetable *hwpt,
>>>>> +				      struct iommufd_device *idev)
>>>>> +{
>>>>> +	struct iommufd_attach_handle *handle;
>>>>> +	int rc;
>>>>> +
>>>>> +	if (hwpt->fault) {
>>>>> +		rc = iommufd_fault_domain_attach_dev(hwpt, idev, true);
>>>>> +		if (rc)
>>>>> +			return rc;
>>>>> +	}
>>>>> +
>>>>> +	handle = kzalloc(sizeof(*handle), GFP_KERNEL);
>>>>> +	if (!handle) {
>>>>> +		rc = -ENOMEM;
>>>>> +		goto out_fault_detach;
>>>>> +	}
>>>>> +
>>>>> +	handle->idev = idev;
>>>>> +	rc = iommu_attach_group_handle(hwpt->domain, idev->igroup->group,
>>>>> +				       &handle->handle);
>>>>> +	if (rc)
>>>>> +		goto out_free_handle;
>>>>> +
>>>>> +	return 0;
>>>>> +
>>>>> +out_free_handle:
>>>>> +	kfree(handle);
>>>>> +	handle = NULL;
>>>>> +out_fault_detach:
>>>>> +	if (hwpt->fault)
>>>>> +		iommufd_fault_domain_detach_dev(hwpt, idev, handle, true);
>>>>> +	return rc;
>>>>> +}
>>>
>>> Here the revert path passes in a handle=NULL..
>>
>> aha. got it. Perhaps we can allocate handle first. In the below thread, it
>> is possible that a failed domain may have pending PRIs, it would require
>> the caller to call the auto response. Although, we are likely to swap the
>> order, but it is nice to have for the caller to do it.
>>
>> https://lore.kernel.org/linux-iommu/f685daca-081a-4ede-b1e1-559009fa9ebc@intel.com/
> 
> Hmm, I don't really see a point in letting the detach flow to
> scan the two lists in hwpt->fault against a zero-ed handle...
> which feels like a waste of CPU cycles?

I meant you may call iommufd_fault_domain_attach_dev() after allocating
handle. Then in the error path, the handle is not zeroed when calling
iommufd_fault_domain_detach_dev(). The cpu circle will not be wasted if
if the two lists are empty. But it would be required if the lists are not
empty. :)

> And I am not sure how that xa_insert part is realted?

Maybe I failed to make it clear. That thread had discussed a case that the
PRIs may be forwarded to hwpt before the attach succeeds. But it is needed
to flush the PRIs in htwp->fault. Although I would swap the order of
xa_insert() and __iommu_set_group_pasid(), it is still nice if iommufd side
flushes the two lists in the error handling path.

Regards,
Yi Liu