[Xen-changelog] [xen stable-4.10] x86: allow Meltdown band-aid to be disabled

[patchbot@xxxxxxx]

commit 7cccd6f748ec724cf9408cec6b3ec8e54a8a2c1f
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Jan 17 16:57:33 2018 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jan 17 16:57:33 2018 +0100

    x86: allow Meltdown band-aid to be disabled
    
    First of all we don't need it on AMD systems. Additionally allow its use
    to be controlled by command line option. For best backportability, this
    intentionally doesn't use alternative instruction patching to achieve
    the intended effect - while we likely want it, this will be later
    follow-up.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: e871e80c38547d9faefc6604532ba3e985e65873
    master date: 2018-01-16 17:50:59 +0100
---
 docs/misc/xen-command-line.markdown | 12 ++++++++++++
 xen/arch/x86/domain.c               |  7 +++++--
 xen/arch/x86/mm.c                   |  2 +-
 xen/arch/x86/smpboot.c              | 17 ++++++++++++++---
 xen/arch/x86/x86_64/entry.S         |  2 ++
 5 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 781110d..49539b4 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1849,6 +1849,18 @@ In the case that x2apic is in use, this option switches 
between physical and
 clustered mode.  The default, given no hint from the **FADT**, is cluster
 mode.
 
+### xpti
+> `= <boolean>`
+
+> Default: `false` on AMD hardware
+> Default: `true` everywhere else
+
+Override default selection of whether to isolate 64-bit PV guest page
+tables.
+
+** WARNING: Not yet a complete isolation implementation, but better than
+nothing. **
+
 ### xsave
 > `= <boolean>`
 
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index f4a3d74..b357b60 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1505,10 +1505,13 @@ void paravirt_ctxt_switch_from(struct vcpu *v)
 
 void paravirt_ctxt_switch_to(struct vcpu *v)
 {
+    root_pgentry_t *root_pgt = this_cpu(root_pgt);
     unsigned long cr4;
 
-    this_cpu(root_pgt)[root_table_offset(PERDOMAIN_VIRT_START)] =
-        l4e_from_page(v->domain->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
+    if ( root_pgt )
+        root_pgt[root_table_offset(PERDOMAIN_VIRT_START)] =
+            l4e_from_page(v->domain->arch.perdomain_l3_pg,
+                          __PAGE_HYPERVISOR_RW);
 
     cr4 = pv_guest_cr4_to_real_cr4(v);
     if ( unlikely(cr4 != read_cr4()) )
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 6c7d120..53295f8 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3665,7 +3665,7 @@ long do_mmu_update(
                     rc = mod_l4_entry(va, l4e_from_intpte(req.val), mfn,
                                       cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
                     if ( !rc )
-                        sync_guest = true;
+                        sync_guest = this_cpu(root_pgt);
                     break;
                 case PGT_writable_page:
                     perfc_incr(writable_mmu_updates);
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index b1fbb57..edf607f 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -328,7 +328,7 @@ void start_secondary(void *unused)
     spin_debug_disable();
 
     get_cpu_info()->xen_cr3 = 0;
-    get_cpu_info()->pv_cr3 = __pa(this_cpu(root_pgt));
+    get_cpu_info()->pv_cr3 = this_cpu(root_pgt) ? __pa(this_cpu(root_pgt)) : 0;
 
     load_system_tables();
 
@@ -736,14 +736,20 @@ static int clone_mapping(const void *ptr, root_pgentry_t 
*rpt)
     return 0;
 }
 
+static __read_mostly int8_t opt_xpti = -1;
+boolean_param("xpti", opt_xpti);
 DEFINE_PER_CPU(root_pgentry_t *, root_pgt);
 
 static int setup_cpu_root_pgt(unsigned int cpu)
 {
-    root_pgentry_t *rpt = alloc_xen_pagetable();
+    root_pgentry_t *rpt;
     unsigned int off;
     int rc;
 
+    if ( !opt_xpti )
+        return 0;
+
+    rpt = alloc_xen_pagetable();
     if ( !rpt )
         return -ENOMEM;
 
@@ -987,10 +993,14 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 
     stack_base[0] = stack_start;
 
+    if ( opt_xpti < 0 )
+        opt_xpti = boot_cpu_data.x86_vendor != X86_VENDOR_AMD;
+
     rc = setup_cpu_root_pgt(0);
     if ( rc )
         panic("Error %d setting up PV root page table\n", rc);
-    get_cpu_info()->pv_cr3 = __pa(per_cpu(root_pgt, 0));
+    if ( per_cpu(root_pgt, 0) )
+        get_cpu_info()->pv_cr3 = __pa(per_cpu(root_pgt, 0));
 
     set_nr_sockets();
 
@@ -1062,6 +1072,7 @@ void __init smp_prepare_boot_cpu(void)
 #endif
 
     get_cpu_info()->xen_cr3 = 0;
+    get_cpu_info()->pv_cr3 = 0;
 }
 
 static void
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index fc38874..a8825c8 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -46,6 +46,7 @@ restore_all_guest:
         movabs $DIRECTMAP_VIRT_START, %rcx
         mov   %rdi, %rax
         and   %rsi, %rdi
+        jz    .Lrag_keep_cr3
         and   %r9, %rsi
         add   %rcx, %rdi
         add   %rcx, %rsi
@@ -62,6 +63,7 @@ restore_all_guest:
         rep movsq
         mov   %r9, STACK_CPUINFO_FIELD(xen_cr3)(%rdx)
         write_cr3 rax, rdi, rsi
+.Lrag_keep_cr3:
 
         RESTORE_ALL
         testw $TRAP_syscall,4(%rsp)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.10

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