x86/pv: Fix assertions in svm_load_segs()

OSSTest has shown an assertion failure:
http://logs.test-lab.xenproject.org/osstest/logs/153906/test-xtf-amd64-amd64-1/serial-rimava1.log

This is because we pass a non-NUL selector into svm_load_segs(), which is
something we must not do, as this path does not load the attributes/limits
from the GDT/LDT.

Drop the {fs,gs}_sel parameters from svm_load_segs() and use 0 instead.  This
is acceptable even for non-zero NUL segments, as it is how the IRET
instruction behaves in all CPUs.

Only use the svm_load_segs() path when both FS and GS are NUL, which is the
common case when scheduling a 64bit vcpu with 64bit userspace in context.

Fixes: ad0fd291c5 ("x86/pv: Rewrite segment context switching from scratch")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/domain.c

@@ -1557,7 +1557,7 @@ static void load_segments(struct vcpu *n)
                    : [_val] "rm" (val) )
 
 #ifdef CONFIG_HVM
-    if ( cpu_has_svm && !compat )
+    if ( cpu_has_svm && !compat && (uregs->fs | uregs->gs) <= 3 )
     {
         unsigned long gsb = n->arch.flags & TF_kernel_mode
             ? n->arch.pv.gs_base_kernel : n->arch.pv.gs_base_user;
@@ -1565,8 +1565,7 @@ static void load_segments(struct vcpu *n)
             ? n->arch.pv.gs_base_user : n->arch.pv.gs_base_kernel;
 
         fs_gs_done = svm_load_segs(n->arch.pv.ldt_ents, LDT_VIRT_START(n),
-                                   uregs->fs, n->arch.pv.fs_base,
-                                   uregs->gs, gsb, gss);
+                                   n->arch.pv.fs_base, gsb, gss);
     }
 #endif
     if ( !fs_gs_done )
@@ -1929,7 +1928,7 @@ static void __context_switch(void)
     /* Prefetch the VMCB if we expect to use it later in the context switch */
     if ( cpu_has_svm && is_pv_domain(nd) && !is_pv_32bit_domain(nd) &&
          !is_idle_domain(nd) )
-        svm_load_segs(0, 0, 0, 0, 0, 0, 0);
+        svm_load_segs(0, 0, 0, 0, 0);
 #endif
 
     if ( need_full_gdt(nd) && !per_cpu(full_gdt_loaded, cpu) )

xen/arch/x86/hvm/svm/svm.c

@@ -1521,8 +1521,7 @@ static void svm_init_erratum_383(const struct cpuinfo_x86 *c)
 
 #ifdef CONFIG_PV
 bool svm_load_segs(unsigned int ldt_ents, unsigned long ldt_base,
-                   unsigned int fs_sel, unsigned long fs_base,
-                   unsigned int gs_sel, unsigned long gs_base,
+                   unsigned long fs_base, unsigned long gs_base,
                    unsigned long gs_shadow)
 {
     unsigned int cpu = smp_processor_id();
@@ -1559,14 +1558,12 @@ bool svm_load_segs(unsigned int ldt_ents, unsigned long ldt_base,
         vmcb->ldtr.base = ldt_base;
     }
 
-    ASSERT(!(fs_sel & ~3));
-    vmcb->fs.sel = fs_sel;
+    vmcb->fs.sel = 0;
     vmcb->fs.attr = 0;
     vmcb->fs.limit = 0;
     vmcb->fs.base = fs_base;
 
-    ASSERT(!(gs_sel & ~3));
-    vmcb->gs.sel = gs_sel;
+    vmcb->gs.sel = 0;
     vmcb->gs.attr = 0;
     vmcb->gs.limit = 0;
     vmcb->gs.base = gs_base;

xen/include/asm-x86/hvm/svm/svm.h

@@ -52,10 +52,12 @@ void svm_update_guest_cr(struct vcpu *, unsigned int cr, unsigned int flags);
 /*
  * PV context switch helper. Calls with zero ldt_base request a prefetch of
  * the VMCB area to be loaded from, instead of an actual load of state.
+ *
+ * Must only be used for NUL FS/GS, as the segment attributes/limits are not
+ * read from the GDT/LDT.
  */
 bool svm_load_segs(unsigned int ldt_ents, unsigned long ldt_base,
-                   unsigned int fs_sel, unsigned long fs_base,
-                   unsigned int gs_sel, unsigned long gs_base,
+                   unsigned long fs_base, unsigned long gs_base,
                    unsigned long gs_shadow);
 
 extern u32 svm_feature_flags;