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[Xen-devel] Re: [RFC PATCH 19/35] subarch support for control register a

To: Chris Wright <chrisw@xxxxxxxxxxxx>
Subject: [Xen-devel] Re: [RFC PATCH 19/35] subarch support for control register accesses
From: Zachary Amsden <zach@xxxxxxxxxx>
Date: Wed, 22 Mar 2006 00:55:12 -0800
Cc: virtualization@xxxxxxxxxxxxxx, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>
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Chris Wright wrote:
+#define read_cr4_safe() ({                           \
+       unsigned int __dummy;                         \
+       /* This could fault if %cr4 does not exist */ \
+       __asm__("1: movl %%cr4, %0         \n"   \
+               "2:                                \n"   \
+               ".section __ex_table,\"a\"       \n"   \
+               ".long 1b,2b                       \n"   \
+               ".previous                 \n"   \
+               : "=r" (__dummy): "0" (0));       \
+       __dummy;                                      \
+})

I think you'll find trap and emulate quite sufficient for this one.

+#define stts() write_cr0(8 | read_cr0())

Nit: You shouldn't need to redefine stts() in the subarch.

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