[Xen-devel] [PATCH 5/5] x86/msr: Fix handling of MSR_AMD_PATCHLEVEL/MSR_IA32_UCODE_REV

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

There are a number of bugs.  There are no read/write hooks on the HVM side, so
guest accesses fall into the "read/write-discard" defaults, which bypass the
correct faulting behaviour and the Intel special case.

For the PV side, writes are discarded (again, bypassing proper faulting),
except for a pinned dom0, which is permitted to actually write the values
other than 0.  This is pointless with read hook implementing the Intel special
case.

However, implementing the Intel special case is itself pointless.  First of
all, OS software can't guarentee to read back 0 in the first place, because a)
this behaviour isn't guarenteed in the SDM, and b) there are SMM handlers
which use the CPUID instruction.  Secondly, when a guest executes CPUID, this
doesn't typically result in Xen executing a CPUID instruction in practice.

With the dom0 special case removed, there are now no writes to this MSR other
than Xen's microcode loading facilities, which means that the value held in
the MSR will be properly up-to-date.  Forward it directly, without jumping
through any hoops.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

v2:
 * Rebase over X86_VENDOR_* bitmap series, to simplify the guest_rdmsr()
   expression.

The migration case is complicated.  A guest which re-evaluates its idea of the
world may find something completely different, but this is probably less bad
letting it see the wrong microcode version.  The cross-vendor case is even
worse, because Intel and AMD report the version in different 32bit words in
this MSR.  The result here is fairly close to current behaviour.
---
 xen/arch/x86/msr.c             | 34 ++++++++++++++++++++++++++++++++++
 xen/arch/x86/pv/emul-priv-op.c | 22 ----------------------
 2 files changed, 34 insertions(+), 22 deletions(-)

diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 4df4a59..3f299af 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -135,6 +135,27 @@ int guest_rdmsr(const struct vcpu *v, uint32_t msr, 
uint64_t *val)
         /* Not offered to guests. */
         goto gp_fault;
 
+    case MSR_AMD_PATCHLEVEL:
+        BUILD_BUG_ON(MSR_IA32_UCODE_REV != MSR_AMD_PATCHLEVEL);
+        /*
+         * AMD and Intel use the same MSR for the current microcode version.
+         *
+         * There is no need to jump through the SDM-provided hoops for Intel.
+         * A guest might itself perform the "write 0, CPUID, read" sequence,
+         * but servicing the CPUID for the guest typically wont result in
+         * actually executing a CPUID instruction.
+         *
+         * As a guest can't influence the value of this MSR, the value will be
+         * from Xen's last microcode load, which can be forwarded straight to
+         * the guest.
+         */
+        if ( !(cp->x86_vendor & (X86_VENDOR_INTEL |X86_VENDOR_AMD)) ||
+             !(boot_cpu_data.x86_vendor &
+               (X86_VENDOR_INTEL | X86_VENDOR_AMD)) ||
+             rdmsr_safe(MSR_AMD_PATCHLEVEL, *val) )
+            goto gp_fault;
+        break;
+
     case MSR_SPEC_CTRL:
         if ( !cp->feat.ibrsb )
             goto gp_fault;
@@ -236,6 +257,19 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
         /* Not offered to guests. */
         goto gp_fault;
 
+    case MSR_AMD_PATCHLEVEL:
+        BUILD_BUG_ON(MSR_IA32_UCODE_REV != MSR_AMD_PATCHLEVEL);
+        /*
+         * AMD and Intel use the same MSR for the current microcode version.
+         *
+         * Both document it as read-only.  However Intel also document that,
+         * for backwards compatiblity, the OS should write 0 to it before
+         * trying to access the current microcode version.
+         */
+        if ( d->arch.cpuid->x86_vendor != X86_VENDOR_INTEL || val != 0 )
+            goto gp_fault;
+        break;
+
     case MSR_AMD_PATCHLOADER:
         /*
          * See note on MSR_IA32_UCODE_WRITE below, which may or may not apply
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index f594065..a55a400 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -893,17 +893,6 @@ static int read_msr(unsigned int reg, uint64_t *val,
         *val = 0;
         return X86EMUL_OKAY;
 
-    case MSR_IA32_UCODE_REV:
-        BUILD_BUG_ON(MSR_IA32_UCODE_REV != MSR_AMD_PATCHLEVEL);
-        if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL )
-        {
-            if ( wrmsr_safe(MSR_IA32_UCODE_REV, 0) )
-                break;
-            /* As documented in the SDM: Do a CPUID 1 here */
-            cpuid_eax(1);
-        }
-        goto normal;
-
     case MSR_IA32_MISC_ENABLE:
         rdmsrl(reg, *val);
         *val = guest_misc_enable(*val);
@@ -1047,17 +1036,6 @@ static int write_msr(unsigned int reg, uint64_t val,
             return X86EMUL_OKAY;
         break;
 
-    case MSR_IA32_UCODE_REV:
-        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL )
-            break;
-        if ( !is_hwdom_pinned_vcpu(curr) )
-            return X86EMUL_OKAY;
-        if ( rdmsr_safe(reg, temp) )
-            break;
-        if ( val )
-            goto invalid;
-        return X86EMUL_OKAY;
-
     case MSR_IA32_MISC_ENABLE:
         rdmsrl(reg, temp);
         if ( val != guest_misc_enable(temp) )
-- 
2.1.4


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