[PATCH 03/22] x86/dom0: only disable SMAP for the PV dom0 build

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The PVH dom0 builder doesn't switch page tables and has no need to run with
SMAP disabled.

Put the SMAP disabling close to the code region where it's necessary, as it
then becomes obvious why switch_cr3_cr4() is required instead of
write_ptbase().

Note removing SMAP from cr4_pv32_mask is not required, as we never jump into
guest context, and hence updating the value of cr4_pv32_mask is not relevant.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/pv/dom0_build.c | 13 ++++++++++---
 xen/arch/x86/setup.c         | 17 -----------------
 2 files changed, 10 insertions(+), 20 deletions(-)

diff --git a/xen/arch/x86/pv/dom0_build.c b/xen/arch/x86/pv/dom0_build.c
index d8043fa58a27..41772dbe80bf 100644
--- a/xen/arch/x86/pv/dom0_build.c
+++ b/xen/arch/x86/pv/dom0_build.c
@@ -370,6 +370,7 @@ int __init dom0_construct_pv(struct domain *d,
     unsigned long alloc_epfn;
     unsigned long initrd_pfn = -1, initrd_mfn = 0;
     unsigned long count;
+    unsigned long cr4;
     struct page_info *page = NULL;
     unsigned int flush_flags = 0;
     start_info_t *si;
@@ -814,8 +815,14 @@ int __init dom0_construct_pv(struct domain *d,
     /* Set up CR3 value for switch_cr3_cr4(). */
     update_cr3(v);
 
+    /*
+     * Temporarily clear SMAP in CR4 to allow user-accesses when running with
+     * the dom0 page-tables.  Cache the value of CR4 so it can be restored.
+     */
+    cr4 = read_cr4();
+
     /* We run on dom0's page tables for the final part of the build process. */
-    switch_cr3_cr4(cr3_pa(v->arch.cr3), read_cr4());
+    switch_cr3_cr4(cr3_pa(v->arch.cr3), cr4 & ~X86_CR4_SMAP);
     mapcache_override_current(v);
 
     /* Copy the OS image and free temporary buffer. */
@@ -836,7 +843,7 @@ int __init dom0_construct_pv(struct domain *d,
              (parms.virt_hypercall >= v_end) )
         {
             mapcache_override_current(NULL);
-            switch_cr3_cr4(current->arch.cr3, read_cr4());
+            switch_cr3_cr4(current->arch.cr3, cr4);
             printk("Invalid HYPERCALL_PAGE field in ELF notes.\n");
             return -EINVAL;
         }
@@ -978,7 +985,7 @@ int __init dom0_construct_pv(struct domain *d,
 
     /* Return to idle domain's page tables. */
     mapcache_override_current(NULL);
-    switch_cr3_cr4(current->arch.cr3, read_cr4());
+    switch_cr3_cr4(current->arch.cr3, cr4);
 
     update_domain_wallclock_time(d);
 
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index eee20bb1753c..bc387d96b519 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -955,26 +955,9 @@ static struct domain *__init create_dom0(const module_t 
*image,
         }
     }
 
-    /*
-     * Temporarily clear SMAP in CR4 to allow user-accesses in 
construct_dom0().
-     * This saves a large number of corner cases interactions with
-     * copy_from_user().
-     */
-    if ( cpu_has_smap )
-    {
-        cr4_pv32_mask &= ~X86_CR4_SMAP;
-        write_cr4(read_cr4() & ~X86_CR4_SMAP);
-    }
-
     if ( construct_dom0(d, image, headroom, initrd, cmdline) != 0 )
         panic("Could not construct domain 0\n");
 
-    if ( cpu_has_smap )
-    {
-        write_cr4(read_cr4() | X86_CR4_SMAP);
-        cr4_pv32_mask |= X86_CR4_SMAP;
-    }
-
     return d;
 }
 
-- 
2.45.2