linux-kernel - Re: [PATCH 1/2] amd/iommu: Preserve domain ids inside the kdump kernel

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Message-ID: <5167fc71-23e2-486e-b9f6-f11960725b19@amd.com>
Date: Thu, 20 Nov 2025 10:32:32 +0530
From: Sairaj Kodilkar <sarunkod@....com>
To: Jason Gunthorpe <jgg@...dia.com>, Vasant Hegde <vasant.hegde@....com>
CC: <iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<joro@...tes.org>, <suravee.suthikulpanit@....com>, <ashish.kalra@....com>,
	<robin.murphy@....com>, <will@...nel.org>
Subject: Re: [PATCH 1/2] amd/iommu: Preserve domain ids inside the kdump
 kernel



On 11/19/2025 7:13 PM, Jason Gunthorpe wrote:
> On Wed, Nov 19, 2025 at 04:20:28PM +0530, Vasant Hegde wrote:
>>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>>> index f2991c11867c..9375fba1071c 100644
>>> --- a/drivers/iommu/amd/init.c
>>> +++ b/drivers/iommu/amd/init.c
>>> @@ -1136,9 +1136,13 @@ static void set_dte_bit(struct dev_table_entry *dte, u8 bit)
>>>   static bool __reuse_device_table(struct amd_iommu *iommu)
>>>   {
>>>   	struct amd_iommu_pci_seg *pci_seg = iommu->pci_seg;
>>> -	u32 lo, hi, old_devtb_size;
>>> +	struct dev_table_entry *old_dev_tbl_entry;
>>> +	u32 lo, hi, old_devtb_size, devid;
>>>   	phys_addr_t old_devtb_phys;
>>> +	u16 dom_id;
>>> +	bool dte_v;
>>>   	u64 entry;
>>> +	int ret;
>>>   
>>>   	/* Each IOMMU use separate device table with the same size */
>>>   	lo = readl(iommu->mmio_base + MMIO_DEV_TABLE_OFFSET);
>>> @@ -1173,6 +1177,25 @@ static bool __reuse_device_table(struct amd_iommu *iommu)
>>>   		return false;
>>>   	}
>>>   
>>> +	for (devid = 0; devid <= pci_seg->last_bdf; devid++) {
>>> +		old_dev_tbl_entry = &pci_seg->old_dev_tbl_cpy[devid];
>>> +		dte_v = old_dev_tbl_entry->data[0] & DTE_FLAG_V;
>>> +		dom_id = old_dev_tbl_entry->data[1] & DEV_DOMID_MASK;
>>> +
>>> +		if (!dte_v || !dom_id)
>>> +			continue;
>>> +		/*
>>> +		 * ID reseveration can fail with -ENOSPC when there
>>> +		 * are multiple devices present in the same domain,
>>> +		 * hence check only for -ENOMEM.
>>> +		 */
>>> +		ret = ida_alloc_range(&pdom_ids, dom_id, dom_id, GFP_ATOMIC);
> Is it really an atomic context? Why?

No, its not atomic context. My intension was to ensure that the memory 
allocation
succeeds. But I am not sure if it's the most appropriate choice for this 
scenario

Could you please suggest which GFP flag would be best suited for this use ?


>
>>> +		if (ret == -ENOMEM) {
>>> +			pr_err("Failed to reserve domain ID 0x%x\n", dom_id);
>>> +			return false;
> Please don't print on ENOMEM, there is already a print.
>
> I think you should also keep iterating as other dom_ids may still be
> fit in already allocated bitmaps. Though the system is probably toast
> if this happens anyhow.

Sure


Thanks
Sairaj
> Jason