Re: [PATCH] disable CPU side GART accesses

[Ingo Molnar]

(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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