[PATCH 4/7] x86/pv: set/clear guest GDT mappings using populate_perdomain_mapping()

[George Dunlap <gwd@xxxxxxxxxxxxxx>]

From: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Until the previous patch, update_xen_slot_in_full_gdt() used the
stashed pointer in d->arch.pv.gdt_ldt_l1tab to update the incoming
vCPU's page tables with Xen's GDT; this was necessary because
perdomain pagetables were mapped from the domheap by default, and
map_domain_page() couldn't be called in a context switch.  Having a
handy pointer to an always-mapped version of the GDT/LDT L1 table,
other sites which modify the table started using it for convenience,
even if they weren't called from within a context switch.  One example
is pv_{set,destroy}_gdt.

Now that all perdomain pagetables are allocated from the xenheap and
their root L3 stashed in d->arch.perdomain_l3,
d->arch.pv.gdt_ldt_l1tab is redundant.  The previous patch removed one
user by modifying update_xen_slot_in_full_gdt() to call
populate_perdomain_mapping().  Continue that process by switching both
pv_{set,destroy}_gdt() to it as well.

pv_destroy_gdt() currently loops over the L1 entries directly,
extracting the MFN from each, dropping the type and reference unless
it was the zero page, and replacing the entry with a read-only mapping
of the zero page.  Since we no longer have the L1 to hand, drop the
references using v->arch.pv.gdt_frames[] instead, and install the
zero-page mappings with a single populate_perdomain_mapping() call.
This makes gdt_frames[] consistently the source of truth for MFNs.

Behaviour is unchanged: torn-down slots map the zero page read-only,
as they have since cf6d39f819 ("x86/PV: properly populate descriptor
tables"), so that LAR/LSL/VERR/VERW on a selector beyond the guest's
limit clear ZF as on native rather than taking a #PF-converted #GP --
and as every PV vCPU's unused slots do from the start, pv_set_gdt()
tearing down the old GDT (zero page included) before installing the
new one.

In the case of pv_set_gdt, we have a slightly awkward situation with
types.  The ABI with the guest uses unsigned long[], but
populate_perdomain_mapping wants an array of mfn_t.
v->arch.pv.gdt_frames being unsigned long means we can just copy from
it across the guest ABI with no conversions.  We could in theory
convert it to mfn_t[] instead, and then pass v->arch.pv.gdt_frames
into populate_perdomain_mapping; but then we'd need to add a
conversion on all the places where frames are copied out.  We choose
instead to copy frames into a temporary mfn_t array on the stack to
pass into populate_perdomain_mapping.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Assisted-by: Claude Code:claude-fable-5, Claude Code:claude-opus-4-8
Signed-off-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
---
Changes since the previously posted version:
- Retain the gdt_ents zeroing when tearing down the GDT (its removal
   was queried by Jan).
- Map torn-down slots read-only to the zero page (via the
   populate_perdomain_mapping() flags parameter) rather than removing
   the mappings with destroy_perdomain_mapping(): empty slots would be
   a guest-visible partial revert of cf6d39f819 (see the commit
   message).  With the destroy call gone, its v->arch.cr3 guard --
   also queried by Jan -- goes too: the zero-page rewrite runs
   unconditionally.
- Keep gdt_frames[] as unsigned long[] rather than switching it to
   mfn_t[] as Jan suggested; the commit message explains the
   trade-off.
- Retitle: destroy_perdomain_mapping() is no longer used here.
---
 xen/arch/x86/pv/descriptor-tables.c | 37 ++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/pv/descriptor-tables.c 
b/xen/arch/x86/pv/descriptor-tables.c
index 8a32b9ae5c..5dda5bffe3 100644
--- a/xen/arch/x86/pv/descriptor-tables.c
+++ b/xen/arch/x86/pv/descriptor-tables.c
@@ -49,33 +49,42 @@ bool pv_destroy_ldt(struct vcpu *v)
 
 void pv_destroy_gdt(struct vcpu *v)
 {
-    l1_pgentry_t *pl1e = pv_gdt_ptes(v);
-    mfn_t zero_mfn = _mfn(virt_to_mfn(zero_page));
-    l1_pgentry_t zero_l1e = l1e_from_mfn(zero_mfn, __PAGE_HYPERVISOR_RO);
+    const mfn_t zero_mfn = _mfn(virt_to_mfn(zero_page));
+    mfn_t zero_mfns[ARRAY_SIZE(v->arch.pv.gdt_frames)];
     unsigned int i;
 
     ASSERT(v == current || !vcpu_cpu_dirty(v));
 
     v->arch.pv.gdt_ents = 0;
-    for ( i = 0; i < FIRST_RESERVED_GDT_PAGE; i++ )
+
+    for ( i = 0; i < ARRAY_SIZE(zero_mfns); i++ )
     {
-        mfn_t mfn = l1e_get_mfn(pl1e[i]);
+        zero_mfns[i] = zero_mfn;
 
-        if ( (l1e_get_flags(pl1e[i]) & _PAGE_PRESENT) &&
-             !mfn_eq(mfn, zero_mfn) )
-            put_page_and_type(mfn_to_page(mfn));
+        /* MFN 0 can never pass get_page_and_type(), so 0 marks unused slots. 
*/
+        if ( !v->arch.pv.gdt_frames[i] )
+            continue;
 
-        l1e_write(&pl1e[i], zero_l1e);
+        put_page_and_type(mfn_to_page(_mfn(v->arch.pv.gdt_frames[i])));
         v->arch.pv.gdt_frames[i] = 0;
     }
+
+    /*
+     * Point every slot at the zero page, read-only: a descriptor fetch from
+     * the unused part of the GDT then finds a not-present descriptor rather
+     * than a missing mapping, so LAR/LSL/VERR/VERW on a selector beyond the
+     * guest's limit clear ZF as they do on native, instead of faulting.
+     */
+    populate_perdomain_mapping(v, GDT_VIRT_START(v), zero_mfns,
+                               ARRAY_SIZE(zero_mfns), __PAGE_HYPERVISOR_RO);
 }
 
 int pv_set_gdt(struct vcpu *v, const unsigned long frames[],
                unsigned int entries)
 {
     struct domain *d = v->domain;
-    l1_pgentry_t *pl1e;
     unsigned int i, nr_frames = DIV_ROUND_UP(entries, 512);
+    mfn_t mfns[ARRAY_SIZE(v->arch.pv.gdt_frames)];
 
     ASSERT(v == current || !vcpu_cpu_dirty(v));
 
@@ -90,6 +99,8 @@ int pv_set_gdt(struct vcpu *v, const unsigned long frames[],
         if ( !mfn_valid(mfn) ||
              !get_page_and_type(mfn_to_page(mfn), d, PGT_seg_desc_page) )
             goto fail;
+
+        mfns[i] = mfn;
     }
 
     /* Tear down the old GDT. */
@@ -97,12 +108,10 @@ int pv_set_gdt(struct vcpu *v, const unsigned long 
frames[],
 
     /* Install the new GDT. */
     v->arch.pv.gdt_ents = entries;
-    pl1e = pv_gdt_ptes(v);
     for ( i = 0; i < nr_frames; i++ )
-    {
         v->arch.pv.gdt_frames[i] = frames[i];
-        l1e_write(&pl1e[i], l1e_from_pfn(frames[i], __PAGE_HYPERVISOR_RW));
-    }
+    populate_perdomain_mapping(v, GDT_VIRT_START(v), mfns, nr_frames,
+                               __PAGE_HYPERVISOR_RW);
 
     return 0;
 
-- 
2.55.0