[Xen-devel] [PATCH 2/4] x86/setup: Minor cleanup to host SYSCALL MSR handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Xen's choice of the MSR_STAR value is constant across all pcpus.  Introduce a
new define and use it to avoid the opencoding in subarch_percpu_traps_init()
and restore_rest_processor_state().

Despite Intel hardware having an MSR_CSTAR register, nothing actually uses it
as the SYSCALL instruction raises #UD out of 64bit mode, meaning that
MSR_LSTAR is the only %rip value hardware will use.

Therefore, only create a CSTAR trampoline stub, and save/restore the CSTAR
value across suspend/resume on AMD hardware.  MSR_CSTAR should now be
consistently 0 on Intel hardware.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/acpi/suspend.c     | 14 ++++++++++----
 xen/arch/x86/x86_64/traps.c     | 17 ++++++++++-------
 xen/include/asm-x86/processor.h |  3 +++
 3 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/acpi/suspend.c b/xen/arch/x86/acpi/suspend.c
index d5c67ee..37fde7d 100644
--- a/xen/arch/x86/acpi/suspend.c
+++ b/xen/arch/x86/acpi/suspend.c
@@ -32,8 +32,11 @@ void save_rest_processor_state(void)
     saved_fs_base = rdfsbase();
     saved_gs_base = rdgsbase();
     rdmsrl(MSR_SHADOW_GS_BASE, saved_kernel_gs_base);
-    rdmsrl(MSR_CSTAR, saved_cstar);
     rdmsrl(MSR_LSTAR, saved_lstar);
+
+    if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+        rdmsrl(MSR_CSTAR, saved_cstar);
+
     if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL ||
          boot_cpu_data.x86_vendor == X86_VENDOR_CENTAUR )
     {
@@ -53,9 +56,12 @@ void restore_rest_processor_state(void)
 
     /* Recover syscall MSRs */
     wrmsrl(MSR_LSTAR, saved_lstar);
-    wrmsrl(MSR_CSTAR, saved_cstar);
-    wrmsr(MSR_STAR, 0, (FLAT_RING3_CS32<<16) | __HYPERVISOR_CS);
-    wrmsr(MSR_SYSCALL_MASK, XEN_SYSCALL_MASK, 0U);
+
+    if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+        wrmsrl(MSR_CSTAR, saved_cstar);
+
+    wrmsrl(MSR_STAR, XEN_MSR_STAR);
+    wrmsrl(MSR_SYSCALL_MASK, XEN_SYSCALL_MASK);
 
     wrfsbase(saved_fs_base);
     wrgsbase(saved_gs_base);
diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index fc8cde6..5afe286 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -403,11 +403,14 @@ void subarch_percpu_traps_init(void)
         wrmsr(MSR_IA32_SYSENTER_CS, __HYPERVISOR_CS, 0);
     }
 
-    /* Trampoline for SYSCALL entry from compatibility mode. */
-    wrmsrl(MSR_CSTAR, stub_va);
-    offset += write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
-                                    stub_va, stack_bottom,
-                                    (unsigned long)cstar_enter);
+    if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+    {
+        /* Trampoline for SYSCALL entry from compatibility mode. */
+        wrmsrl(MSR_CSTAR, stub_va);
+        offset += write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
+                                        stub_va, stack_bottom,
+                                        (unsigned long)cstar_enter);
+    }
 
     /* Don't consume more than half of the stub space here. */
     ASSERT(offset <= STUB_BUF_SIZE / 2);
@@ -415,8 +418,8 @@ void subarch_percpu_traps_init(void)
     unmap_domain_page(stub_page);
 
     /* Common SYSCALL parameters. */
-    wrmsr(MSR_STAR, 0, ((unsigned int)FLAT_RING3_CS32 << 16) | 
__HYPERVISOR_CS);
-    wrmsr(MSR_SYSCALL_MASK, XEN_SYSCALL_MASK, 0U);
+    wrmsrl(MSR_STAR, XEN_MSR_STAR);
+    wrmsrl(MSR_SYSCALL_MASK, XEN_SYSCALL_MASK);
 }
 
 void init_int80_direct_trap(struct vcpu *v)
diff --git a/xen/include/asm-x86/processor.h b/xen/include/asm-x86/processor.h
index 7735bc2..843f072 100644
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -87,6 +87,9 @@
 
 #define XEN_CR4_PV32_BITS (X86_CR4_SMEP|X86_CR4_SMAP)
 
+/* Common SYSCALL parameters. */
+#define XEN_MSR_STAR (((uint64_t)FLAT_RING3_CS32 << 48) |   \
+                      ((uint64_t)__HYPERVISOR_CS << 32))
 #define XEN_SYSCALL_MASK (X86_EFLAGS_AC|X86_EFLAGS_VM|X86_EFLAGS_RF|    \
                           X86_EFLAGS_NT|X86_EFLAGS_DF|X86_EFLAGS_IF|    \
                           X86_EFLAGS_TF)
-- 
2.1.4


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