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SubjectRe: [PATCH] disable CPU side GART accesses

(Cc:-ed the GART folks.)

* Bob Montgomery <> wrote:

> This patch prevents improper access of the GART aperture from kdump
> kernels running on AMD systems.
> Symptoms of the problem include hangs, spurious restarts, and MCE
> (Machine Check Exception) panics in some AMD Opteron systems that
> enable the GART IOMMU and access /proc/vmcore or /dev/oldmem from a
> kdump kernel. Note that the GART IOMMU will not be enabled on systems
> with less than 4 GB of RAM, so symptoms will not appear. This problem
> has been reproduced on Family 10H Quad-Core AMD Opteron systems.
> This patch changes the initialization of the GART to set the
> DISGARTCPU bit in the GART Aperture Control Register
> (AMD64_GARTAPERTURECTL). Setting the bit prevents requests from the
> CPUs from accessing the GART. In other words, CPU memory accesses to
> the aperture address range will not cause the GART to perform an
> address translation. The aperture area is currently being unmapped at
> the kernel level with set_memory_np() in gart_iommu_init to prevent
> accesses from the CPU, but that kernel level unmapping is not in
> effect in the kexec'd kdump kernel. By disabling the CPU-side
> accesses within the GART, which does persist through the kexec of the
> kdump kernel, the kdump kernel is prevented from interacting with the
> GART during accesses to the dump memory areas which include the
> address range of the GART aperture. Although the patch can be applied
> to the kdump kernel, it is not exercised there because the kdump
> kernel doesn't attempt to initialize the GART, since it typically runs
> in less than 4 GB of memory.
> Signed-off-by: Bob Montgomery <>
> --- linux-2.6.27/include/asm-x86/gart.h 2008-10-13 16:36:34.000000000 -0600
> +++ linux-2.6.27-fix/include/asm-x86/gart.h 2008-10-14 10:37:32.000000000 -0600
> @@ -44,7 +44,8 @@ static inline void enable_gart_translati
> /* Enable GART translation for this hammer. */
> pci_read_config_dword(dev, AMD64_GARTAPERTURECTL, &ctl);
> ctl |= GARTEN;
> + ctl |= DISGARTCPU;
> + ctl &= ~(DISGARTIO);
> pci_write_config_dword(dev, AMD64_GARTAPERTURECTL, ctl);
> }
> --
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