Re: [PATCH] x86/mem-sharing: short-circuit p2m_is_shared() when MEM_SHARING=n

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.04.2025 16:50, Andrew Cooper wrote:
> On 03/04/2025 9:41 am, Jan Beulich wrote:
>> Some of the uses of dom_cow aren't easily DCE-able (without extra
>> #ifdef-ary), and hence it being constantly NULL when MEM_SHARING=n
>> misguides Coverity into thinking that there may be a NULL deref in
>>
>>         if ( p2m_is_shared(t) )
>>             d = dom_cow;
>>
>>         if ( get_page(page, d) )
>>             return page;
>>
>> (in get_page_from_mfn_and_type()). Help the situation by making
>> p2m_is_shared() be compile-time false when MEM_SHARING=n, thus also
>> permitting the compiler to DCE some other code.
>>
>> Note that p2m_is_sharable() isn't used outside of mem_sharing.c, and
>> hence P2M_SHARABLE_TYPES can simply be left undefined when
>> MEM_SHARING=n.

Note this for ...

>> --- a/xen/arch/x86/include/asm/p2m.h
>> +++ b/xen/arch/x86/include/asm/p2m.h
>> @@ -136,11 +136,16 @@ typedef unsigned int p2m_query_t;
>>  #endif
>>  
>>  /* Shared types */
>> +#ifdef CONFIG_MEM_SHARING
>>  /* XXX: Sharable types could include p2m_ram_ro too, but we would need to
>>   * reinit the type correctly after fault */
>>  #define P2M_SHARABLE_TYPES (p2m_to_mask(p2m_ram_rw) \
>>                              | p2m_to_mask(p2m_ram_logdirty) )
>>  #define P2M_SHARED_TYPES   (p2m_to_mask(p2m_ram_shared))
>> +#else
>> +/* P2M_SHARABLE_TYPES deliberately not provided. */
>> +#define P2M_SHARED_TYPES 0
> 
> You need P2M_SHARABLE_TYPES too, or p2m_is_sharable() will start
> becoming a syntax error.

... what you're saying here. If you did take that into consideration, then
please be more specific about the concern(s) you have.

Jan