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

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Message-ID: <20251119134351.GM120075@nvidia.com>
Date: Wed, 19 Nov 2025 09:43:51 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Vasant Hegde <vasant.hegde@....com>
Cc: Sairaj Kodilkar <sarunkod@....com>, 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 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?

> > +		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.

Jason