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

[patchbot@xxxxxxx]

commit 516e89a43bc67d86b83f539c97b7ffc6d6aa19d6
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Dec 31 16:52:39 2024 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Mar 17 16:08:34 2025 +0000

    x86/boot: Fix zap_low_mappings() to map less of the trampoline
    
    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>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 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 389d813ebe..d4e6a9c0a2 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)
--
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