linux-kernel - [PATCH 54 of 55] xen: implement Xen write

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Date:	Tue, 08 Jul 2008 15:07:16 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	Stephen Tweedie <sct@...hat.com>,
	Eduardo Habkost <ehabkost@...hat.com>,
	Mark McLoughlin <markmc@...hat.com>, x86@...nel.org
Subject: [PATCH 54 of 55] xen: implement Xen write_msr operation

64-bit uses MSRs for important things like the base for fs and
gs-prefixed addresses.  It's more efficient to use a hypercall to
update these, rather than go via the trap and emulate path.

Other MSR writes are just passed through; in an unprivileged domain
they do nothing, but it might be useful later.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
---
 arch/x86/xen/enlighten.c |   31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -41,6 +41,7 @@
 #include <asm/xen/hypervisor.h>
 #include <asm/fixmap.h>
 #include <asm/processor.h>
+#include <asm/msr-index.h>
 #include <asm/setup.h>
 #include <asm/desc.h>
 #include <asm/pgtable.h>
@@ -777,6 +778,34 @@
 	xen_mc_issue(PARAVIRT_LAZY_CPU);  /* interrupts restored */
 }
 
+static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
+{
+	int ret;
+
+	ret = 0;
+
+	switch(msr) {
+#ifdef CONFIG_X86_64
+		unsigned which;
+		u64 base;
+
+	case MSR_FS_BASE:		which = SEGBASE_FS; goto set;
+	case MSR_KERNEL_GS_BASE:	which = SEGBASE_GS_USER; goto set;
+	case MSR_GS_BASE:		which = SEGBASE_GS_KERNEL; goto set;
+
+	set:
+		base = ((u64)high << 32) | low;
+		if (HYPERVISOR_set_segment_base(which, base) != 0)
+			ret = -EFAULT;
+		break;
+#endif
+	default:
+		ret = native_write_msr_safe(msr, low, high);
+	}
+
+	return ret;
+}
+
 /* Early in boot, while setting up the initial pagetable, assume
    everything is pinned. */
 static __init void xen_alloc_pte_init(struct mm_struct *mm, u32 pfn)
@@ -1165,7 +1194,7 @@
 	.wbinvd = native_wbinvd,
 
 	.read_msr = native_read_msr_safe,
-	.write_msr = native_write_msr_safe,
+	.write_msr = xen_write_msr_safe,
 	.read_tsc = native_read_tsc,
 	.read_pmc = native_read_pmc,
 


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