[RFC PATCH 03/22] x86/msr: always allow a pinned Dom0 to read any unknown MSR

[Edwin Török <edwin.torok@xxxxxxxxx>]

From: Edwin Török <edvin.torok@xxxxxxxxxx>

This can be useful if you realize you have to inspect the value of an
MSR in production, without having to change into a new Xen first that
handles the MSR.

E.g. SMI count didn't use to be explicitly allowed in the past
(it now is, see a previous commit), but there could be other MSRs that
are useful when tracking down issues.

Backport: 4.15+

Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
---
 xen/arch/x86/hvm/svm/svm.c     | 3 +++
 xen/arch/x86/hvm/vmx/vmx.c     | 3 +++
 xen/arch/x86/pv/emul-priv-op.c | 3 +++
 3 files changed, 9 insertions(+)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 24c417ca71..45f8e1ffd1 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1933,6 +1933,9 @@ static int cf_check svm_msr_read_intercept(
         break;
 
     default:
+        if ( is_hwdom_pinned_vcpu(v) && !rdmsr_safe(msr, *msr_content) )
+            break;
+
         if ( d->arch.msr_relaxed && !rdmsr_safe(msr, tmp) )
         {
             *msr_content = 0;
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 1edc7f1e91..f6e5123f66 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3377,6 +3377,9 @@ static int cf_check vmx_msr_read_intercept(
         if ( vmx_read_guest_msr(curr, msr, msr_content) == 0 )
             break;
 
+        if ( is_hwdom_pinned_vcpu(curr) && !rdmsr_safe(msr, *msr_content) )
+            return X86EMUL_OKAY;
+
         if ( is_last_branch_msr(msr) )
         {
             *msr_content = 0;
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 0d9f84f458..978ae679a2 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -985,6 +985,9 @@ static int cf_check read_msr(
         }
         /* fall through */
     default:
+        if ( is_hwdom_pinned_vcpu(curr) && !rdmsr_safe(reg, *val) )
+            return X86EMUL_OKAY;
+
         if ( currd->arch.msr_relaxed && !rdmsr_safe(reg, tmp) )
         {
             *val = 0;
-- 
2.41.0