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Subject[PATCH 54 of 55] xen: implement Xen write_msr operation
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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@citrix.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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