[Xen-changelog] [xen stable-4.8] x86: allow Meltdown band-aid to be disabled

[patchbot@xxxxxxx]

commit 31d38d633a306b2b06767b5a5f5a8a00269f3c92
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Jan 17 17:17:26 2018 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jan 17 17:17:26 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 0fcdb7d..768d4f5 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1687,6 +1687,18 @@ 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 bf3590d..8817263 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1947,12 +1947,15 @@ static void paravirt_ctxt_switch_from(struct vcpu *v)
 
 static void paravirt_ctxt_switch_to(struct vcpu *v)
 {
+    root_pgentry_t *root_pgt = this_cpu(root_pgt);
     unsigned long cr4;
 
     switch_kernel_stack(v);
 
-    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 69e1ab6..303c551 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4011,7 +4011,7 @@ 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 327c744..c19508f 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -320,7 +320,7 @@ 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();
 
@@ -729,14 +729,20 @@ 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;
 
@@ -977,10 +983,14 @@ 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();
 
@@ -1048,6 +1058,7 @@ void __init smp_prepare_boot_cpu(void)
     cpumask_set_cpu(smp_processor_id(), &cpu_present_map);
 
     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 d1afb3c..505604f 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -45,6 +45,7 @@ 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
@@ -61,6 +62,7 @@ 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-4.8

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