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iommu/amd: Don't allocate kdump device table from DMA32

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Booting a kdump kernel with crashkernel=X,high crashkernel=0,low on an
AMD system fails the per-segment device table allocation because there
is no memory below 4G:

  swapper/0: page allocation failure: order:9, mode:0x40104(GFP_DMA32|__GFP_ZERO|__GFP_COMP)
  iommu_alloc_pages_node_sz+0x7f/0x150
  alloc_pci_segment+0x4bd/0x650
  iommu_go_to_state+0x213d/0x34e0
  amd_iommu_prepare+0x39/0xa0
  irq_remapping_prepare+0x83/0xd0
  enable_IR_x2apic+0x61/0x370

On our E810 systems, the kdump kernel booted but ice MSI-X interrupts
never fired and the TX queues timed out.

Commit b336781b82 ("iommu/amd: Allocate memory below 4G for dev table
if translation pre-enabled") allocated device tables below 4G. At the
time, the kdump kernel programmed a copy of the old table into the base
register with the IOMMU still enabled. The 64-bit register is updated
as two 32-bit halves, so the running IOMMU could see a half-written,
invalid table pointer. Keeping both tables below 4G leaves the upper
half zero throughout the update, so the pointer stays valid.

Since commit 38e5f33ee3 ("iommu/amd: Reuse device table for kdump")
the kdump kernel reuses the previous kernel's table without writing the
base register, so its newly allocated table is discarded when reuse
succeeds. If reuse fails, the previous patch programs the new table only
after disabling the IOMMU. Either way, the kdump kernel's table doesn't
need to be below 4G.

The previous kernel still allocates its table from DMA32, so the kdump
kernel's check that the old table is below 4G still works. Drop
GFP_DMA32 for kdump kernels only.

Cc: stable@vger.kernel.org # 6.18.x
Reviewed-by: Ashish Kalra <ashish.kalra@amd.com>
Reviewed-by: Ankit Soni <Ankit.Soni@amd.com>
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Reviewed-by: Ashish Kalra <ashish.kalra@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
This commit is contained in:
Matt Fleming 2026-10-13 13:19:44 +01:00 committed by Joerg Roedel
parent 05d62be400
commit bae0d37606
1 changed files with 13 additions and 2 deletions
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15
drivers/iommu/amd/init.c
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@ -628,8 +628,19 @@ static int __init find_last_devid_acpi(struct acpi_table_header *table, u16 pci_
/* Allocate per PCI segment device table */
static inline int __init alloc_dev_table(struct amd_iommu_pci_seg *pci_seg)
{
pci_seg->dev_table = iommu_alloc_pages_sz(GFP_KERNEL | GFP_DMA32,
pci_seg->dev_table_size);
gfp_t gfp = GFP_KERNEL;
/*
* Keep normal kernel device tables below 4G because a later kdump
* kernel only trusts and reuses old tables below that limit. A kdump
* kernel's new table is either discarded when reuse succeeds or
* programmed after the IOMMU is disabled, so the table does not
* need DMA32 memory (which the crashkernel may not have).
*/
if (!is_kdump_kernel())
gfp |= GFP_DMA32;
pci_seg->dev_table = iommu_alloc_pages_sz(gfp, pci_seg->dev_table_size);
if (!pci_seg->dev_table)
return -ENOMEM;