[PATCH] x86/boot: Fix zap_low_mappings() to map less of the trampoline

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Regular data access into the trampoline is via the directmap.

As now discussed quite extensively in asm/trampoline.h, the trampoline is
arranged so that only the AP and S3 paths need an identity mapping, and that
they fit within a single page.

Right now, PFN_UP(trampoline_end - trampoline_start) is 2, causing more than
expected of the trampoline to be mapped.  Cut it down just the single page it
ought to be.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

There's not an obvious candidate for a Fixes tag.
---
 xen/arch/x86/x86_64/mm.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index 389d813ebe63..d4e6a9c0a2e0 100644
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -718,14 +718,16 @@ void __init zap_low_mappings(void)
 {
     BUG_ON(num_online_cpus() != 1);
 
-    /* Remove aliased mapping of first 1:1 PML4 entry. */
+    /* Stop using l?_bootmap[] mappings. */
     l4e_write(&idle_pg_table[0], l4e_empty());
     flush_local(FLUSH_TLB_GLOBAL);
 
-    /* Replace with mapping of the boot trampoline only. */
+    /*
+     * Insert an identity mapping of the AP/S3 part of the trampoline, which
+     * is arranged to fit in a single page.
+     */
     map_pages_to_xen(trampoline_phys, maddr_to_mfn(trampoline_phys),
-                     PFN_UP(trampoline_end - trampoline_start),
-                     __PAGE_HYPERVISOR_RX);
+                     1, __PAGE_HYPERVISOR_RX);
 }
 
 int setup_compat_arg_xlat(struct vcpu *v)
-- 
2.39.5