x86/HVM: correct notion of new CPL in task switch emulation

Commit aac1df3d03 ("x86/HVM: introduce hvm_get_cpl() and respective
hook") went too far in one aspect: When emulating a task switch we
really shouldn't be looking at what hvm_get_cpl() returns, as we're
switching all segment registers.

However, instead of reverting the relevant parts of that commit, have
the caller tell the segment loading function what the new CPL is. This
at once fixes ES being loaded before CS so far having had its checks
done against the old CPL.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
An alternative to adding yet another parameter to the function would
be to have "cpl" have a special case value (e.g. negative) to indicate
VM86 mode. That would allow replacing the current "eflags" parameter.

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2616,11 +2616,11 @@ static void hvm_unmap_entry(void *p)
 }
 
 static int hvm_load_segment_selector(
-    enum x86_segment seg, uint16_t sel, unsigned int eflags)
+    enum x86_segment seg, uint16_t sel, unsigned int cpl, unsigned int eflags)
 {
     struct segment_register desctab, segr;
     struct desc_struct *pdesc, desc;
-    u8 dpl, rpl, cpl;
+    u8 dpl, rpl;
     bool_t writable;
     int fault_type = TRAP_invalid_tss;
     struct vcpu *v = current;
@@ -2674,7 +2674,6 @@ static int hvm_load_segment_selector(
 
         dpl = (desc.b >> 13) & 3;
         rpl = sel & 3;
-        cpl = hvm_get_cpl(v);
 
         switch ( seg )
         {
@@ -2804,7 +2803,7 @@ void hvm_task_switch(
     struct segment_register gdt, tr, prev_tr, segr;
     struct desc_struct *optss_desc = NULL, *nptss_desc = NULL, tss_desc;
     bool_t otd_writable, ntd_writable;
-    unsigned int eflags;
+    unsigned int eflags, new_cpl;
     pagefault_info_t pfinfo;
     int exn_raised, rc;
     struct tss32 tss;
@@ -2918,7 +2917,9 @@ void hvm_task_switch(
     if ( rc != HVMCOPY_okay )
         goto out;
 
-    if ( hvm_load_segment_selector(x86_seg_ldtr, tss.ldt, 0) )
+    new_cpl = tss.eflags & X86_EFLAGS_VM ? 3 : tss.cs & 3;
+
+    if ( hvm_load_segment_selector(x86_seg_ldtr, tss.ldt, new_cpl, 0) )
         goto out;
 
     rc = hvm_set_cr3(tss.cr3, 1);
@@ -2939,12 +2940,12 @@ void hvm_task_switch(
     regs->rdi    = tss.edi;
 
     exn_raised = 0;
-    if ( hvm_load_segment_selector(x86_seg_es, tss.es, tss.eflags) ||
-         hvm_load_segment_selector(x86_seg_cs, tss.cs, tss.eflags) ||
-         hvm_load_segment_selector(x86_seg_ss, tss.ss, tss.eflags) ||
-         hvm_load_segment_selector(x86_seg_ds, tss.ds, tss.eflags) ||
-         hvm_load_segment_selector(x86_seg_fs, tss.fs, tss.eflags) ||
-         hvm_load_segment_selector(x86_seg_gs, tss.gs, tss.eflags) )
+    if ( hvm_load_segment_selector(x86_seg_es, tss.es, new_cpl, tss.eflags) ||
+         hvm_load_segment_selector(x86_seg_cs, tss.cs, new_cpl, tss.eflags) ||
+         hvm_load_segment_selector(x86_seg_ss, tss.ss, new_cpl, tss.eflags) ||
+         hvm_load_segment_selector(x86_seg_ds, tss.ds, new_cpl, tss.eflags) ||
+         hvm_load_segment_selector(x86_seg_fs, tss.fs, new_cpl, tss.eflags) ||
+         hvm_load_segment_selector(x86_seg_gs, tss.gs, new_cpl, tss.eflags) )
         exn_raised = 1;
 
     if ( taskswitch_reason == TSW_call_or_int )