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@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 30 insertions, 1 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index f64b8729cd07..776c0fb77d69 100644
--- 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 @@ static void xen_write_cr3(unsigned long cr3)
         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 @@ static const struct pv_cpu_ops xen_cpu_ops __initdata = {
         .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,