Re: [PATCH] disable CPU side GART accesses
- From: Ingo Molnar <mingo@xxxxxxx>
- Date: Thu, 16 Oct 2008 01:48:42 +0200
(Cc:-ed the GART folks.)
* Bob Montgomery <bob.montgomery@xxxxxx> 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 <bob.montgomery@xxxxxx>
--- 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 | DISGARTIO);
+ ctl |= DISGARTCPU;
+ ctl &= ~(DISGARTIO);
pci_write_config_dword(dev, AMD64_GARTAPERTURECTL, ctl);
}
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