[Xen-changelog] [xen stable-4.7] x86: allow Meltdown band-aid to be disabled

[patchbot@xxxxxxx]

commit e19d0af4ee2ae9e42a85db639fd6848e72f5658b
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Jan 17 17:24:59 2018 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jan 17 17:24:59 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 2dacb5d..aecf9fd 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1621,6 +1621,18 @@ 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 3cf18f9..a1bda5e 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1945,12 +1945,15 @@ 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 c9e4003..07015e3 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4007,7 +4007,7 @@ 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 = 1;
+                        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 eaeec5a..f2f47f6 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 d63e734..2a56995 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-4.7

_______________________________________________
Xen-changelog mailing list
Xen-changelog@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/xen-changelog