[xen stable-4.12] x86: make hypervisor build with gcc11

[patchbot@xxxxxxx]

commit bd52c7e902f4ae20386d6b02c7394ca49d601571
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Aug 25 15:43:57 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Aug 25 15:43:57 2021 +0200

    x86: make hypervisor build with gcc11
    
    Gcc 11 looks to make incorrect assumptions about valid ranges that
    pointers may be used for addressing when they are derived from e.g. a
    plain constant. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100680.
    
    Utilize RELOC_HIDE() to work around the issue, which for x86 manifests
    in at least
    - mpparse.c:efi_check_config(),
    - tboot.c:tboot_probe(),
    - tboot.c:tboot_gen_frametable_integrity(),
    - x86_emulate.c:x86_emulate() (at -O2 only).
    The last case is particularly odd not just because it only triggers at
    higher optimization levels, but also because it only affects one of at
    least three similar constructs. Various "note" diagnostics claim the
    valid index range to be [0, 2�³-1].
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Tested-by: Jason Andryuk <jandryuk@xxxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: 722f59d38c710a940ab05e542a83020eb5546dea
    master date: 2021-05-27 14:40:29 +0200
---
 tools/tests/x86_emulator/x86-emulate.c | 7 +++++++
 xen/arch/x86/x86_emulate/x86_emulate.c | 2 +-
 xen/include/asm-x86/fixmap.h           | 2 +-
 xen/include/xen/compiler.h             | 6 ++++++
 xen/include/xen/pdx.h                  | 2 +-
 5 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/tools/tests/x86_emulator/x86-emulate.c 
b/tools/tests/x86_emulator/x86-emulate.c
index 8132fe7a7c..dc098c9e06 100644
--- a/tools/tests/x86_emulator/x86-emulate.c
+++ b/tools/tests/x86_emulator/x86-emulate.c
@@ -2,6 +2,13 @@
 
 #include <sys/mman.h>
 
+/* See gcc bug 100680, but here don't bother making this version dependent. */
+#define gcc11_wrap(x) ({                  \
+    unsigned long x_;                     \
+    __asm__ ( "" : "=g" (x_) : "0" (x) ); \
+    (typeof(x))x_;                        \
+})
+
 #define cpu_has_amd_erratum(nr) 0
 #define cpu_has_mpx false
 #define read_bndcfgu() 0
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index c7c55fa572..88456bec9e 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -644,7 +644,7 @@ union vex {
 #define copy_VEX(ptr, vex) ({ \
     if ( !mode_64bit() ) \
         (vex).reg |= 8; \
-    (ptr)[0 - PFX_BYTES] = ext < ext_8f08 ? 0xc4 : 0x8f; \
+    gcc11_wrap(ptr)[0 - PFX_BYTES] = ext < ext_8f08 ? 0xc4 : 0x8f; \
     (ptr)[1 - PFX_BYTES] = (vex).raw[0]; \
     (ptr)[2 - PFX_BYTES] = (vex).raw[1]; \
     container_of((ptr) + 1 - PFX_BYTES, typeof(vex), raw[0]); \
diff --git a/xen/include/asm-x86/fixmap.h b/xen/include/asm-x86/fixmap.h
index 16ccaa2c77..d460b71f4f 100644
--- a/xen/include/asm-x86/fixmap.h
+++ b/xen/include/asm-x86/fixmap.h
@@ -80,7 +80,7 @@ extern void __set_fixmap(
 
 #define clear_fixmap(idx) __set_fixmap(idx, 0, 0)
 
-#define __fix_to_virt(x) (FIXADDR_TOP - ((x) << PAGE_SHIFT))
+#define __fix_to_virt(x) gcc11_wrap(FIXADDR_TOP - ((x) << PAGE_SHIFT))
 #define __virt_to_fix(x) ((FIXADDR_TOP - ((x)&PAGE_MASK)) >> PAGE_SHIFT)
 
 #define fix_to_virt(x)   ((void *)__fix_to_virt(x))
diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index ff6c0f5cdd..b1c0374dfe 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -99,6 +99,12 @@
     __asm__ ("" : "=r"(__ptr) : "0"(ptr));      \
     (typeof(ptr)) (__ptr + (off)); })
 
+#if __GNUC__ >= 11 /* See gcc bug 100680. */
+# define gcc11_wrap(x) RELOC_HIDE(x, 0)
+#else
+# define gcc11_wrap(x) (x)
+#endif
+
 #ifdef __GCC_ASM_FLAG_OUTPUTS__
 # define ASM_FLAG_OUT(yes, no) yes
 #else
diff --git a/xen/include/xen/pdx.h b/xen/include/xen/pdx.h
index a151aac1a2..5ed51b5edb 100644
--- a/xen/include/xen/pdx.h
+++ b/xen/include/xen/pdx.h
@@ -19,7 +19,7 @@ extern u64 pdx_region_mask(u64 base, u64 len);
 extern void set_pdx_range(unsigned long smfn, unsigned long emfn);
 
 #define page_to_pdx(pg)  ((pg) - frame_table)
-#define pdx_to_page(pdx) (frame_table + (pdx))
+#define pdx_to_page(pdx) gcc11_wrap(frame_table + (pdx))
 
 bool __mfn_valid(unsigned long mfn);
 
--
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