Re: [PATCH v2 2/2] iommu/amd: Don't allocate kdump device table from DMA32

[Vasant Hegde <vasant.hegde@amd.com>]

On 10/13/2026 5:49 PM, Matt Fleming wrote:
> From: Matt Fleming <mfleming@cloudflare.com>
> 
> 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 b336781b82cc ("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 38e5f33ee359 ("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.

This one is OK as its regression fix. But with all the changes went in I am
wondering why we need GFP_DMA32 flag for allocation. There are only two cases
where transactions may be enabled:
  - During boot (BIOS enabled DMA) : kernel will disable IOMMU before
configuring device table base address w/ new value
  - During kdump : We are reusing previous kernels device table. If it fails
then kernel will disable IOMMU and re-configures it with new vlaue.

Anyway, we will look into it in detail and run more experiments. For now this
fix is fine. With that,

Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>

-Vasant