[PATCH 5/5] x86/pv: Simplify emulation for the 64bit base MSRs

[Andrew Cooper]

is_pv_32bit_domain() is an expensive predicate, but isn't used for speculative
safety in this case.  Swap to checking the Long Mode bit in the CPUID policy,
which is the architecturally correct behaviour.

is_canonical_address() isn't a trivial predicate, but it will become more
complicated when 5-level support is added.  Rearrange write_msr() to collapse
the common checks.

Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
---
CC: Jan Beulich <jbeul...@suse.com>
CC: Roger Pau Monné <roger....@citrix.com>
CC: Wei Liu <w...@xen.org>

For reference, the diff is:

  add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-152 (-152)
  Function                                     old     new   delta
  read_msr                                    1075    1030     -45
  write_msr                                   1537    1430    -107

but this isn't the point of the change.
---
 xen/arch/x86/pv/emul-priv-op.c | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 9dd1d59423..0fd95fe9fa 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -848,6 +848,7 @@ static int read_msr(unsigned int reg, uint64_t *val,
 {
     struct vcpu *curr = current;
     const struct domain *currd = curr->domain;
+    const struct cpuid_policy *cp = currd->arch.cpuid;
     bool vpmu_msr = false;
     int ret;
 
@@ -869,19 +870,19 @@ static int read_msr(unsigned int reg, uint64_t *val,
         return X86EMUL_OKAY;
 
     case MSR_FS_BASE:
-        if ( is_pv_32bit_domain(currd) )
+        if ( !cp->extd.lm )
             break;
         *val = read_fs_base();
         return X86EMUL_OKAY;
 
     case MSR_GS_BASE:
-        if ( is_pv_32bit_domain(currd) )
+        if ( !cp->extd.lm )
             break;
         *val = read_gs_base();
         return X86EMUL_OKAY;
 
     case MSR_SHADOW_GS_BASE:
-        if ( is_pv_32bit_domain(currd) )
+        if ( !cp->extd.lm )
             break;
         *val = curr->arch.pv.gs_base_user;
         return X86EMUL_OKAY;
@@ -975,6 +976,7 @@ static int write_msr(unsigned int reg, uint64_t val,
 {
     struct vcpu *curr = current;
     const struct domain *currd = curr->domain;
+    const struct cpuid_policy *cp = currd->arch.cpuid;
     bool vpmu_msr = false;
     int ret;
 
@@ -991,22 +993,22 @@ static int write_msr(unsigned int reg, uint64_t val,
         uint64_t temp;
 
     case MSR_FS_BASE:
-        if ( is_pv_32bit_domain(currd) || !is_canonical_address(val) )
-            break;
-        write_fs_base(val);
-        return X86EMUL_OKAY;
-
     case MSR_GS_BASE:
-        if ( is_pv_32bit_domain(currd) || !is_canonical_address(val) )
-            break;
-        write_gs_base(val);
-        return X86EMUL_OKAY;
-
     case MSR_SHADOW_GS_BASE:
-        if ( is_pv_32bit_domain(currd) || !is_canonical_address(val) )
+        if ( !cp->extd.lm || !is_canonical_address(val) )
             break;
-        write_gs_shadow(val);
-        curr->arch.pv.gs_base_user = val;
+
+        if ( reg == MSR_FS_BASE )
+            write_fs_base(val);
+        else if ( reg == MSR_GS_BASE )
+            write_gs_base(val);
+        else if ( reg == MSR_SHADOW_GS_BASE )
+        {
+            write_gs_shadow(val);
+            curr->arch.pv.gs_base_user = val;
+        }
+        else
+            ASSERT_UNREACHABLE();
         return X86EMUL_OKAY;
 
     case MSR_EFER:
-- 
2.11.0