[Xen-changelog] [xen master] xen/arm64: Don't zero BSS when booting using EFI

[patchbot@xxxxxxx]

commit 6cc8e8708e65f42d12ff7ea02156aafc5afdb2f7
Author:     Julien Grall <julien.grall@xxxxxxx>
AuthorDate: Wed Feb 8 10:48:14 2017 +0000
Commit:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
CommitDate: Wed Feb 8 15:32:02 2017 -0800

    xen/arm64: Don't zero BSS when booting using EFI
    
    Commit 146786b "efi: create efi_enabled()" introduced a variable
    efi_flags stored in BSS and used to pass information between the stub
    and Xen. However on ARM, BSS is zeroed after the stub has finished to
    run and before Xen is started. This means that the bits set in efi_flags
    will be lost.
    
    We were not affected before because all the variables used to pass
    information between Xen and the stub are living in initdata or data.
    
    Looking at the description of the field SizeOfRawData in the PE/COFF
    header (see [1]):
    
    "If this is less than VirtualSize, the remainder of the section is
    zero-filled. Because the SizeOfRawData field is rounded but the
    VirtualSize field is not, it is possible for SizeOfRawData to be greater
    than VirtualSize as well. When a section contains only uninitialized
    data, this field should be zero."
    
    Both VirtualSize and SizeOfRawData are correctly set in the header (see
    arch/arm/arm64/head.S) so the EFI firmware will zero BSS for us.
    
    Therefore we don't need to zero BSS before running the EFI stub and can
    skip the one between the EFI stub and Xen.
    
    To avoid another branch instruction, slightly refactor the code. The
    register x26 is allocated to hold whether BSS is skipped. The value will
    be:
        - 0 when the code is running on CPU0 and EFI is not used
        - 1 when EFI is used or running on other processor than the boot one.
    
    [1] 
https://msdn.microsoft.com/en-us/library/windows/desktop/ms680547(v=vs.85).aspx
    
    Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Reviewed-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
---
 xen/arch/arm/arm64/head.S | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 3f63d2a..3a54356 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -65,7 +65,7 @@
  *  x23 - UART address
  *  x24 - cpuid
  *  x25 - identity map in place
- *  x26 -
+ *  x26 - skip_zero_bss
  *  x27 -
  *  x28 -
  *  x29 -
@@ -232,6 +232,10 @@ section_table:
         .long   0xe0500020       /* Characteristics (section flags) */
         .align  5
 real_start:
+        /* BSS should be zeroed when booting without EFI */
+        mov   x26, #0                /* x26 := skip_zero_bss */
+
+real_start_efi:
         msr   DAIFSet, 0xf           /* Disable all interrupts */
 
         /* Save the bootloader arguments in less-clobberable registers */
@@ -261,6 +265,8 @@ GLOBAL(init_secondary)
         sub   x20, x19, x0           /* x20 := phys-offset */
 
         mov   x22, #1                /* x22 := is_secondary_cpu */
+        /* Boot CPU already zero BSS so skip it on secondary CPUs. */
+        mov   x26, #1                /* X26 := skip_zero_bss */
 
 common_start:
         mov   x24, #0                /* x24 := CPU ID. Initialy zero until we
@@ -314,8 +320,8 @@ common_start:
 
 el2:    PRINT("- Xen starting at EL2 -\r\n")
 
-        /* Zero BSS On the boot CPU to avoid nasty surprises */
-        cbnz  x22, skip_bss
+        /* Zero BSS only when requested */
+        cbnz  x26, skip_bss
 
         PRINT("- Zero BSS -\r\n")
         ldr   x0, =__bss_start       /* Load start & end of bss */
@@ -787,7 +793,16 @@ ENTRY(efi_xen_start)
         mov   x1, xzr
         mov   x2, xzr
         mov   x3, xzr
-        b     real_start
+        /*
+         * The EFI stub and Xen may share some information living in
+         * BSS. Don't zero BSS to avoid loosing them.
+         *
+         * Note that the EFI firmware has already zeroed BSS for us
+         * before jump into the stub.
+         */
+        mov   x26, #1               /* x26 := skip_zero_bss */
+
+        b     real_start_efi
 ENDPROC(efi_xen_start)
 
 /*
--
generated by git-patchbot for /home/xen/git/xen.git#master

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