[PATCH 3/7] x86/pv: use populate_perdomain_mapping() to map the Xen GDT

[George Dunlap <gwd@xxxxxxxxxxxxxx>]

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

Currently, update_xen_slot_in_full_gdt() uses the stashed pointer in
d->arch.pv.gdt_ldt_l1tab to update the incoming vcpu's page-tables with
Xen's GDT, by writing a stashed per-cpu copy of a pre-baked L1 entry
(either 64-bit or compat version).

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.  Remove one user by switching
update_xen_slot_in_full_gdt() to using populate_perdomain_mapping().

Since populate_perdomain_mapping() takes an mfn rather than an l1e,
cache the mfn of the per-cpu page instead.  As a side effect, this
consolidates the setting of the flags into a single place.  Continue
to check that the per-cpu value we're using has been initialized:
per-CPU data starts out zeroed, and no GDT can live at MFN 0.

What was one store through a cached pointer is now an out-of-line
walk (L3 pointer, L3e, L2e, then the L1e) on every PV context switch:
three dependent loads, negligible next to the CR3 write that follows.

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:
- populate_perdomain_mapping() introduction split into the previous
   patch; this patch is now just the Xen GDT conversion.
- Retain the "GDT MFN cached" check as ASSERT(mfn_x(mfn)).
---
 xen/arch/x86/domain.c           | 13 +++++++++----
 xen/arch/x86/include/asm/desc.h |  6 ++++--
 xen/arch/x86/smpboot.c          | 14 +++++---------
 xen/arch/x86/traps.c            |  4 ++--
 4 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 163a2c97ae..f5b2eef95a 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2062,11 +2062,16 @@ static always_inline bool need_full_gdt(const struct 
domain *d)
 
 static void update_xen_slot_in_full_gdt(const struct vcpu *v, unsigned int cpu)
 {
-    ASSERT(per_cpu(gdt_l1e, cpu).l1); /* Confirm these have been cached. */
+    mfn_t mfn = !is_pv_32bit_vcpu(v) ? per_cpu(gdt_mfn, cpu)
+                                     : per_cpu(compat_gdt_mfn, cpu);
 
-    l1e_write(pv_gdt_ptes(v) + FIRST_RESERVED_GDT_PAGE,
-              !is_pv_32bit_vcpu(v) ? per_cpu(gdt_l1e, cpu)
-                                   : per_cpu(compat_gdt_l1e, cpu));
+    /* Confirm the GDT MFNs have been cached (MFN 0 is never a GDT). */
+    ASSERT(mfn_x(mfn));
+
+    populate_perdomain_mapping(v,
+                               GDT_VIRT_START(v) +
+                               (FIRST_RESERVED_GDT_PAGE << PAGE_SHIFT),
+                               &mfn, 1, __PAGE_HYPERVISOR_RW);
 }
 
 static void load_full_gdt(const struct vcpu *v, unsigned int cpu)
diff --git a/xen/arch/x86/include/asm/desc.h b/xen/arch/x86/include/asm/desc.h
index dcbdac3ff7..a26df9fbe9 100644
--- a/xen/arch/x86/include/asm/desc.h
+++ b/xen/arch/x86/include/asm/desc.h
@@ -44,6 +44,8 @@
 
 #ifndef __ASSEMBLER__
 
+#include <xen/mm-frame.h>
+
 #define GUEST_KERNEL_RPL(d) (is_pv_32bit_domain(d) ? 1 : 3)
 
 /* Fix up the RPL of a guest segment selector. */
@@ -136,10 +138,10 @@ struct __packed desc_ptr {
 
 extern seg_desc_t boot_gdt[];
 DECLARE_PER_CPU(seg_desc_t *, gdt);
-DECLARE_PER_CPU(l1_pgentry_t, gdt_l1e);
+DECLARE_PER_CPU(mfn_t, gdt_mfn);
 extern seg_desc_t boot_compat_gdt[];
 DECLARE_PER_CPU(seg_desc_t *, compat_gdt);
-DECLARE_PER_CPU(l1_pgentry_t, compat_gdt_l1e);
+DECLARE_PER_CPU(mfn_t, compat_gdt_mfn);
 DECLARE_PER_CPU(bool, full_gdt_loaded);
 
 static inline void lgdt(const struct desc_ptr *gdtr)
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 84e9e4beed..9246945506 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -1085,8 +1085,7 @@ static int cpu_smpboot_alloc(unsigned int cpu)
     if ( gdt == NULL )
         goto out;
     per_cpu(gdt, cpu) = gdt;
-    per_cpu(gdt_l1e, cpu) =
-        l1e_from_pfn(virt_to_mfn(gdt), __PAGE_HYPERVISOR_RW);
+    per_cpu(gdt_mfn, cpu) = _mfn(virt_to_mfn(gdt));
     memcpy(gdt, boot_gdt, NR_RESERVED_GDT_PAGES * PAGE_SIZE);
     BUILD_BUG_ON(NR_CPUS > 0x10000);
     gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
@@ -1095,8 +1094,7 @@ static int cpu_smpboot_alloc(unsigned int cpu)
     per_cpu(compat_gdt, cpu) = gdt = alloc_xenheap_pages(0, memflags);
     if ( gdt == NULL )
         goto out;
-    per_cpu(compat_gdt_l1e, cpu) =
-        l1e_from_pfn(virt_to_mfn(gdt), __PAGE_HYPERVISOR_RW);
+    per_cpu(compat_gdt_mfn, cpu) = _mfn(virt_to_mfn(gdt));
     memcpy(gdt, boot_compat_gdt, NR_RESERVED_GDT_PAGES * PAGE_SIZE);
     gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
 #endif
@@ -1174,15 +1172,13 @@ void __init smp_prepare_cpus(void)
     print_cpu_info(0);
 
     /*
-     * Cache {,compat_}gdt_l1e for the BSP now that physically relocation is
+     * Cache {,compat_}gdt_mfn for the BSP now that physically relocation is
      * done.  It must be after physical relocation of Xen, and before the
      * first context_switch().
      */
-    this_cpu(gdt_l1e) =
-        l1e_from_pfn(virt_to_mfn(boot_gdt), __PAGE_HYPERVISOR_RW);
+    this_cpu(gdt_mfn) = _mfn(virt_to_mfn(boot_gdt));
     if ( IS_ENABLED(CONFIG_PV32) )
-        this_cpu(compat_gdt_l1e) =
-            l1e_from_pfn(virt_to_mfn(boot_compat_gdt), __PAGE_HYPERVISOR_RW);
+        this_cpu(compat_gdt_mfn) = _mfn(virt_to_mfn(boot_compat_gdt));
 
     boot_cpu_physical_apicid = get_apic_id();
     x86_cpu_to_apicid[0] = boot_cpu_physical_apicid;
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 1774966305..2fcaa413f7 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -71,10 +71,10 @@ DEFINE_PER_CPU(uint64_t, efer);
 static DEFINE_PER_CPU(unsigned long, last_extable_addr);
 
 DEFINE_PER_CPU_READ_MOSTLY(seg_desc_t *, gdt);
-DEFINE_PER_CPU_READ_MOSTLY(l1_pgentry_t, gdt_l1e);
+DEFINE_PER_CPU_READ_MOSTLY(mfn_t, gdt_mfn);
 #ifdef CONFIG_PV32
 DEFINE_PER_CPU_READ_MOSTLY(seg_desc_t *, compat_gdt);
-DEFINE_PER_CPU_READ_MOSTLY(l1_pgentry_t, compat_gdt_l1e);
+DEFINE_PER_CPU_READ_MOSTLY(mfn_t, compat_gdt_mfn);
 #endif
 
 /*
-- 
2.55.0