Re: [PATCH] x86: prevent unconditional writes to DebugCtl MSR

[Roland McGrath]

I don't think this was a good idea:

	commit 5b0e508415a83989fe704b4718a1a214bc333ca7
	Author: Jan Beulich <jbeulich@xxxxxxxxxx>
	Date:   Mon Mar 10 13:11:17 2008 +0000

	    x86: prevent unconditional writes to DebugCtl MSR

It's already a bug if there is any unconditional use of the MSR.
Silenting ignoring it is just wrong.

There was such a bug before this fix:

	commit 4ba51fd75cc3789be83f0d6f878dabbb0cb19bca
	Author: Roland McGrath <roland@xxxxxxxxxx>
	Date:   Thu Apr 3 14:18:55 2008 -0700

	    x86 ptrace: avoid unnecessary wrmsr

But there should not be any more.  The use of the MSR for block-step is
controlled by arch_has_block_step(), which uses the same condition.  Any
use of the MSR for other purposes (DS et al) should be controlled by more
specific CPU model checks.  

If TIF_DEBUGCTLMSR is ever set on a machine without the support, that is a
bug we should diagnose earlier.  If you want some paranoia, then keep
update_debugctlmsr but make it do:

	BUG_ON(boot_cpu_data.x86 < 6);

instead.


Thanks,
Roland
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