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Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ] > > From: Matt Fleming > > 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. Keeping the table below 4G means > it doesn't matter that the hardware splits that write into two 32-bit > halves. Probably, expand/explain this a bit: The 64-bit base register is not updated atomically - the write lands as two 32-bit halves - so mid-update the live IOMMU can briefly latch a half-written, invalid table pointer. Allocating the table below 4G keeps the upper 32 bits zero, so the register stays valid throughout the update and the running IOMMU never sees a bad pointer. > > 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. Since dropping GFP_DMA32 is only safe on top of the reuse conversion, i.e., on top of 38e5f33ee359, so it should not be backported without it. Probably, consider added a Cc:stable tag without Fixes: tag, so it doesn't read as if 38e5f33 was buggy Cc: stable@vger.kernel.org # depends on 38e5f33ee359 Signed-off-by: Matt Fleming mfleming@cloudflare.com Reviewed-by: Ashish Kalra ashish.kalra@amd.com > > 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. > > Signed-off-by: Matt Fleming > --- > drivers/iommu/amd/init.c | 15 +++++++++++++-- > 1 file changed, 13 insertions(+), 2 deletions(-) > > diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c > index 0572e1a674f0..16c26d75f46f 100644 > --- a/drivers/iommu/amd/init.c > +++ b/drivers/iommu/amd/init.c > @@ -648,8 +648,19 @@ 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; > > -- > 2.43.0 >