Re: [PATCH] memory: fix build with COVERAGE but !HVM

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/02/2021 16:20, Jan Beulich wrote:
> Xen is heavily relying on the DCE stage to remove unused code so the
> linker doesn't throw an error because a function is not implemented
> yet we defined a prototype for it.
>
> On some GCC versions (such as 9.4 provided by Debian sid), the compiler
> DCE stage will not manage to figure that out for
> xenmem_add_to_physmap_batch():
>
> ld: ld: prelink.o: in function `xenmem_add_to_physmap_batch':
> /xen/xen/common/memory.c:942: undefined reference to 
> `xenmem_add_to_physmap_one'
> /xen/xen/common/memory.c:942:(.text+0x22145): relocation truncated
> to fit: R_X86_64_PLT32 against undefined symbol `xenmem_add_to_physmap_one'
> prelink-efi.o: in function `xenmem_add_to_physmap_batch':
> /xen/xen/common/memory.c:942: undefined reference to 
> `xenmem_add_to_physmap_one'
> make[2]: *** [Makefile:215: /root/xen/xen/xen.efi] Error 1
> make[2]: *** Waiting for unfinished jobs....
> ld: /xen/xen/.xen-syms.0: hidden symbol `xenmem_add_to_physmap_one' isn't 
> defined
> ld: final link failed: bad value
>
> It is not entirely clear why the compiler DCE is not detecting the
> unused code. However, cloning the check introduced by the commit below
> into xenmem_add_to_physmap_batch() does the trick.
>
> No functional change intended.
>
> Fixes: d4f699a0df6c ("x86/mm: p2m_add_foreign() is HVM-only")
> Reported-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>
> ---
> Julien, since I reused most of your patch'es description, I've kept your
> S-o-b. Please let me know if you want me to drop it.
>
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -904,6 +904,19 @@ static int xenmem_add_to_physmap_batch(s
>  {
>      union add_to_physmap_extra extra = {};
>  
> +    /*
> +     * While, unlike xenmem_add_to_physmap(), this function is static, there
> +     * still have been cases observed where xatp_permission_check(), invoked
> +     * by our caller, doesn't lead to elimination of this entire function 
> when
> +     * the compile time evaluation of paging_mode_translate(d) is false. 
> Guard
> +     * against this be replicating the same check here.
> +     */

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, but I feel this
comment can be far more precise/concise.

/* In some configurations, (!HVM, COVERAGE), the
xenmem_add_to_physmap_one() call doesn't succumb to
dead-code-elimination.  Duplicate the short-circut from
xatp_permission_check() to try and help the compiler out. */

~Andrew