Re: [PATCH] x86/altp2m: help gcc13 to avoid it emitting a warning

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/03/2023 7:31 am, Jan Beulich wrote:
> Switches of altp2m-s always expect a valid altp2m to be in place (and
> indeed altp2m_vcpu_initialise() sets the active one to be at index 0).
> The compiler, however, cannot know that, and hence it cannot eliminate
> p2m_get_altp2m()'s case of returnin (literal) NULL. If then the compiler
> decides to special case that code path in the caller, the dereference in
> instances of
>
>     atomic_dec(&p2m_get_altp2m(v)->active_vcpus);
>
> can, to the code generator, appear to be NULL dereferences, leading to
>
> In function 'atomic_dec',
>     inlined from '...' at ...:
> ./arch/x86/include/asm/atomic.h:182:5: error: array subscript 0 is outside 
> array bounds of 'int[0]' [-Werror=array-bounds=]
>
> Aid the compiler by adding a BUG_ON() checking the return value of the
> problematic p2m_get_altp2m(). Since with the use of the local variable
> the 2nd p2m_get_altp2m() each will look questionable at the first glance
> (Why is the local variable not used here?), open-code the only relevant
> piece of p2m_get_altp2m() there.
>
> To avoid repeatedly doing these transformations, and also to limit how
> "bad" the open-coding really is, convert the entire operation to an
> inline helper, used by all three instances (and accepting the redundant
> BUG_ON(idx >= MAX_ALTP2M) in two of the three cases).
>
> Reported-by: Charles Arnold <carnold@xxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>