# Commit cf6d39f81992c29a637c603dbabf1e21a0ea563f
# Date 2015-09-25 09:05:29 +0200
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
x86/PV: properly populate descriptor tables

Us extending the GDT limit past the Xen descriptors so far meant that
guests (including user mode programs) accessing any descriptor table
slot above the original OS'es limit but below the first Xen descriptor
caused a #PF, converted to a #GP in our #PF handler. Which is quite
different from the native behavior, where some of such accesses (LAR
and LSL) don't fault. Mimic that behavior by mapping a blank page into
unused slots.

While not strictly required, treat the LDT the same for consistency.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

# Commit 61031e64d3dafd2fb1953436444bf02eccb9b146
# Date 2015-10-27 14:46:12 +0100
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
x86/PV: don't zero-map LDT

This effectvely reverts the LDT related part of commit cf6d39f819
("x86/PV: properly populate descriptor tables"), which broke demand
paged LDT handling in guests.

Reported-by: David Vrabel <david.vrabel@citrix.com>
Diagnosed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -505,12 +505,12 @@ void update_cr3(struct vcpu *v)
     make_cr3(v, cr3_mfn);
 }
 
+static const char __section(".bss.page_aligned") zero_page[PAGE_SIZE];
 
 static void invalidate_shadow_ldt(struct vcpu *v, int flush)
 {
     l1_pgentry_t *pl1e;
-    int i;
-    unsigned long pfn;
+    unsigned int i;
     struct page_info *page;
 
     BUG_ON(unlikely(in_irq()));
@@ -525,10 +525,10 @@ static void invalidate_shadow_ldt(struct
 
     for ( i = 16; i < 32; i++ )
     {
-        pfn = l1e_get_pfn(pl1e[i]);
-        if ( pfn == 0 ) continue;
+        if ( !(l1e_get_flags(pl1e[i]) & _PAGE_PRESENT) )
+            continue;
+        page = l1e_get_page(pl1e[i]);
         l1e_write(&pl1e[i], l1e_empty());
-        page = mfn_to_page(pfn);
         ASSERT_PAGE_IS_TYPE(page, PGT_seg_desc_page);
         ASSERT_PAGE_IS_DOMAIN(page, v->domain);
         put_page_and_type(page);
@@ -4360,16 +4360,18 @@ long do_update_va_mapping_otherdomain(un
 void destroy_gdt(struct vcpu *v)
 {
     l1_pgentry_t *pl1e;
-    int i;
-    unsigned long pfn;
+    unsigned int i;
+    unsigned long pfn, zero_pfn = PFN_DOWN(__pa(zero_page));
 
     v->arch.pv_vcpu.gdt_ents = 0;
     pl1e = gdt_ldt_ptes(v->domain, v);
     for ( i = 0; i < FIRST_RESERVED_GDT_PAGE; i++ )
     {
-        if ( (pfn = l1e_get_pfn(pl1e[i])) != 0 )
+        pfn = l1e_get_pfn(pl1e[i]);
+        if ( (l1e_get_flags(pl1e[i]) & _PAGE_PRESENT) && pfn != zero_pfn )
             put_page_and_type(mfn_to_page(pfn));
-        l1e_write(&pl1e[i], l1e_empty());
+        l1e_write(&pl1e[i],
+                  l1e_from_pfn(zero_pfn, __PAGE_HYPERVISOR & ~_PAGE_RW));
         v->arch.pv_vcpu.gdt_frames[i] = 0;
     }
 }
@@ -4382,7 +4384,7 @@ long set_gdt(struct vcpu *v,
     struct domain *d = v->domain;
     l1_pgentry_t *pl1e;
     /* NB. There are 512 8-byte entries per GDT page. */
-    int i, nr_pages = (entries + 511) / 512;
+    unsigned int i, nr_pages = (entries + 511) / 512;
 
     if ( entries > FIRST_RESERVED_GDT_ENTRY )
         return -EINVAL;