Re: [PATCH 3/4] Revert "x86: disable IOMMUs on kernel crash"

From: Eric W. Biederman
Date: Sat Apr 03 2010 - 13:44:46 EST

Joerg Roedel <joro@xxxxxxxxxx> writes:

> On Fri, Apr 02, 2010 at 06:27:54PM -0700, Chris Wright wrote:
>> This effectively reverts commit 61d047be99757fd9b0af900d7abce9a13a337488.
>> Disabling the IOMMU can potetially allow DMA transactions to
>> complete without being translated. Leave it enabled, and allow
>> crash kernel to do the IOMMU reinitialization properly.
>> Cc: Joerg Roedel <joerg.roedel@xxxxxxx>
>> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
>> Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
>> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
>> Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
>> ---
>> arch/x86/kernel/crash.c | 6 ------
>> 1 file changed, 6 deletions(-)
>> --- a/arch/x86/kernel/crash.c
>> +++ b/arch/x86/kernel/crash.c
>> @@ -27,7 +27,6 @@
>> #include <asm/cpu.h>
>> #include <asm/reboot.h>
>> #include <asm/virtext.h>
>> -#include <asm/x86_init.h>
>> #if defined(CONFIG_SMP) && defined(CONFIG_X86_LOCAL_APIC)
>> @@ -103,10 +102,5 @@ void native_machine_crash_shutdown(struc
>> hpet_disable();
>> #endif
>> -
>> -#ifdef CONFIG_X86_64
>> - x86_platform.iommu_shutdown();
>> -#endif
>> -
>> crash_save_cpu(regs, safe_smp_processor_id());
> Hmm, I think for this we need to change the gart code too and disable
> the gart before its initialization runs to not re-introduce issues fixed
> in commit bc2cea6a34fdb30f118ec75db39a46a191870607, no?

That is a different code path with a different set of assumptions and
restrictions. On a normal kexec of course we want to do an orderly shutdown.

For the gart with a little luck we can just ignore it on kexec on
panic. Unlike a virtualization capable iommu it doesn't prevent access
to devices, when it is enabled. Worst case is that we have to start
including iommu=off for gart systems. The best case is that we can
figure out how to have the gart code reinitialize itself sanely,
starting from some arbitrary point.

machine_crash_shutdown is about doing those things that we can not do
in any other way.

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