Re: [Xen-devel] [PATCH 2/2] arch/x86/xen: Silence compiler warnings

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Fri, Jul 11, 2014 at 04:32:27PM -0400, Boris Ostrovsky wrote:
> On 07/11/2014 04:10 PM, Daniel Kiper wrote:
> >On Fri, Jul 11, 2014 at 04:03:46PM -0400, Boris Ostrovsky wrote:
> >>On 07/11/2014 03:54 PM, Daniel Kiper wrote:
> >>>Compiler complains in the following way when x86 32-bit kernel
> >>>with Xen support is build:
> >>>
> >>>   CC      arch/x86/xen/enlighten.o
> >>>arch/x86/xen/enlighten.c: In function âxen_start_kernelâ:
> >>>arch/x86/xen/enlighten.c:1726:3: warning: right shift count >= width of 
> >>>type [enabled by default]
> >>>
> >>>Such line contains following EFI initialization code:
> >>>
> >>>boot_params.efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 32);
> >>>
> >>>There is no issue if x86 64-bit kernel is build. However, 32-bit case
> >>>generate warning (even if that code will not be executed because Xen
> >>>does not work on 32-bit EFI platforms) due to __pa() returning unsigned 
> >>>long
> >>>type which has 32-bits width. So move whole EFI initialization stuff
> >>>to separate function and build its body conditionally to avoid above
> >>>mentioned warning on x86 32-bit architecture.
> >>>
> >>>Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> >>>---
> >>>  arch/x86/xen/enlighten.c |   35 ++++++++++++++++++++++-------------
> >>>  1 file changed, 22 insertions(+), 13 deletions(-)
> >>>
> >>>diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> >>>index bc89647..6abec74 100644
> >>>--- a/arch/x86/xen/enlighten.c
> >>>+++ b/arch/x86/xen/enlighten.c
> >>>@@ -1516,12 +1516,32 @@ static void __init xen_pvh_early_guest_init(void)
> >>>  #endif
> >>>  }
> >>>+static void __init xen_efi_init(void)
> >>>+{
> >>>+#ifdef CONFIG_XEN_EFI
> >>>+  efi_system_table_t *efi_systab_xen;
> >>>+
> >>>+  efi_systab_xen = xen_efi_probe();
> >>>+
> >>>+  if (efi_systab_xen == NULL)
> >>>+          return;
> >>>+
> >>>+  strncpy((char *)&boot_params.efi_info.efi_loader_signature, "Xen",
> >>>+                  sizeof(boot_params.efi_info.efi_loader_signature));
> >>>+  boot_params.efi_info.efi_systab = (__u32)__pa(efi_systab_xen);
> >>>+  boot_params.efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 
> >>>32);
> >>>+
> >>>+  set_bit(EFI_BOOT, &efi.flags);
> >>>+  set_bit(EFI_PARAVIRT, &efi.flags);
> >>>+  set_bit(EFI_64BIT, &efi.flags);
> >>>+#endif
> >>>+}
> >>>+
> >>>  /* First C function to be called on Xen boot */
> >>>  asmlinkage __visible void __init xen_start_kernel(void)
> >>>  {
> >>>   struct physdev_set_iopl set_iopl;
> >>>   int rc;
> >>>-  efi_system_table_t *efi_systab_xen;
> >>>   if (!xen_start_info)
> >>>           return;
> >>>@@ -1717,18 +1737,7 @@ asmlinkage __visible void __init 
> >>>xen_start_kernel(void)
> >>>   xen_setup_runstate_info(0);
> >>>-  efi_systab_xen = xen_efi_probe();
> >>>-
> >>>-  if (efi_systab_xen) {
> >>>-          strncpy((char *)&boot_params.efi_info.efi_loader_signature, 
> >>>"Xen",
> >>>-                          
> >>>sizeof(boot_params.efi_info.efi_loader_signature));
> >>>-          boot_params.efi_info.efi_systab = (__u32)__pa(efi_systab_xen);
> >>>-          boot_params.efi_info.efi_systab_hi = 
> >>>(__u32)(__pa(efi_systab_xen) >> 32);
> >>>-
> >>>-          set_bit(EFI_BOOT, &efi.flags);
> >>>-          set_bit(EFI_PARAVIRT, &efi.flags);
> >>>-          set_bit(EFI_64BIT, &efi.flags);
> >>>-  }
> >>>+  xen_efi_init();
> >>I'd put ifdef CONFIG_XEN_EFI around the call instead of having it
> >>inside the routine.
> >Well, I thought about that a bit and I prefer function like Konrad.
> >Could you agree with him which solution do you (as maintainers) prefer?
> >
>
> I am not arguing against having a separate routine. All I am saying
> is that calling xen_efi_init() when CONFIG_XEN_EFI is not defined
> doesn't look logical. It will also add an unnecessary call (although

Ahh... I misunderstood you. However, your proposal, as below:

#ifdef CONFIG_XEN_EFI
  xen_efi_init();
#endif

does not solve the problem because this vulnerable shift will be still
visible for compiler during x86 32-bit kernel build.

> compiler may optimize it out).

Please loot at arch/x86/xen/enlighten.c:xen_check_mwait() and
arch/x86/xen/enlighten.c:xen_boot_params_init_edd() (probably
there are more stuff like that around). As I can see this is fairly
common solution and probably compiler cope with it quite well.

Have a nice weekend,

Daniel

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