[Xen-devel] [PATCH v4 3/5] x86/paravirt: Add paravirt_{read, write}_msr

[Andy Lutomirski <luto@xxxxxxxxxx>]

This adds paravirt hooks for unsafe MSR access.  On native, they
call native_{read,write}_msr.  On Xen, they use
xen_{read,write}_msr_safe.

Nothing uses them yet for ease of bisection.  The next patch will
use them in rdmsrl, wrmsrl, etc.

I intentionally didn't make them OOPS on #GP on Xen.  I think that
should be done separately by the Xen maintainers.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/msr.h            |  5 +++--
 arch/x86/include/asm/paravirt.h       | 11 +++++++++++
 arch/x86/include/asm/paravirt_types.h | 10 ++++++++--
 arch/x86/kernel/paravirt.c            |  2 ++
 arch/x86/xen/enlighten.c              | 23 +++++++++++++++++++++++
 5 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 1487054a1a70..13da359881d7 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -119,8 +119,9 @@ static inline unsigned long long 
native_read_msr_safe(unsigned int msr,
        return EAX_EDX_VAL(val, low, high);
 }
 
-static inline void native_write_msr(unsigned int msr,
-                                   unsigned low, unsigned high)
+/* Can be uninlined because referenced by paravirt */
+notrace static inline void native_write_msr(unsigned int msr,
+                                           unsigned low, unsigned high)
 {
        asm volatile("1: wrmsr\n"
                     "2:\n"
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 2e49228ed9a3..68297d87e85c 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -129,6 +129,17 @@ static inline void wbinvd(void)
 
 #define get_kernel_rpl()  (pv_info.kernel_rpl)
 
+static inline u64 paravirt_read_msr(unsigned msr)
+{
+       return PVOP_CALL1(u64, pv_cpu_ops.read_msr, msr);
+}
+
+static inline void paravirt_write_msr(unsigned msr,
+                                     unsigned low, unsigned high)
+{
+       return PVOP_VCALL3(pv_cpu_ops.write_msr, msr, low, high);
+}
+
 static inline u64 paravirt_read_msr_safe(unsigned msr, int *err)
 {
        return PVOP_CALL2(u64, pv_cpu_ops.read_msr_safe, msr, err);
diff --git a/arch/x86/include/asm/paravirt_types.h 
b/arch/x86/include/asm/paravirt_types.h
index 5a06cccd36f0..b85aec45a1b8 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -155,8 +155,14 @@ struct pv_cpu_ops {
        void (*cpuid)(unsigned int *eax, unsigned int *ebx,
                      unsigned int *ecx, unsigned int *edx);
 
-       /* MSR operations.
-          err = 0/-EIO.  wrmsr returns 0/-EIO. */
+       /* Unsafe MSR operations.  These will warn or panic on failure. */
+       u64 (*read_msr)(unsigned int msr);
+       void (*write_msr)(unsigned int msr, unsigned low, unsigned high);
+
+       /*
+        * Safe MSR operations.
+        * read sets err to 0 or -EIO.  write returns 0 or -EIO.
+        */
        u64 (*read_msr_safe)(unsigned int msr, int *err);
        int (*write_msr_safe)(unsigned int msr, unsigned low, unsigned high);
 
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 8aad95478ae5..f9583917c7c4 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -339,6 +339,8 @@ __visible struct pv_cpu_ops pv_cpu_ops = {
        .write_cr8 = native_write_cr8,
 #endif
        .wbinvd = native_wbinvd,
+       .read_msr = native_read_msr,
+       .write_msr = native_write_msr,
        .read_msr_safe = native_read_msr_safe,
        .write_msr_safe = native_write_msr_safe,
        .read_pmc = native_read_pmc,
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index ff2df20d0067..548cd3e02b9e 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1092,6 +1092,26 @@ static int xen_write_msr_safe(unsigned int msr, unsigned 
low, unsigned high)
        return ret;
 }
 
+static u64 xen_read_msr(unsigned int msr)
+{
+       /*
+        * This will silently swallow a #GP from RDMSR.  It may be worth
+        * changing that.
+        */
+       int err;
+
+       return xen_read_msr_safe(msr, &err);
+}
+
+static void xen_write_msr(unsigned int msr, unsigned low, unsigned high)
+{
+       /*
+        * This will silently swallow a #GP from WRMSR.  It may be worth
+        * changing that.
+        */
+       xen_write_msr_safe(msr, low, high);
+}
+
 void xen_setup_shared_info(void)
 {
        if (!xen_feature(XENFEAT_auto_translated_physmap)) {
@@ -1222,6 +1242,9 @@ static const struct pv_cpu_ops xen_cpu_ops __initconst = {
 
        .wbinvd = native_wbinvd,
 
+       .read_msr = xen_read_msr,
+       .write_msr = xen_write_msr,
+
        .read_msr_safe = xen_read_msr_safe,
        .write_msr_safe = xen_write_msr_safe,
 
-- 
2.5.0


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