Re: [PATCH v3 4/4] x86/shadow: re-work 4-level SHADOW_FOREACH_L2E()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.02.2023 18:26, Andrew Cooper wrote:
> On 08/02/2023 2:38 pm, Jan Beulich wrote:
>> First of all move the almost loop-invariant condition out of the loop;
>> transform it into an altered loop boundary, noting that the updating of
>> _gl2p is relevant only at one use site, and then also only inside the
>> _code blob it provides. Then drop the shadow_mode_external() part of the
>> condition as being redundant with the is_pv_32bit_domain() check.
>> Further, since the new local variable wants to be "unsigned int",
>> convert the loop induction variable accordingly. Finally also adjust
>> formatting as most code needs touching anyway.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks.

>> --- a/xen/arch/x86/mm/shadow/multi.c
>> +++ b/xen/arch/x86/mm/shadow/multi.c
>> @@ -861,23 +861,22 @@ do {
>>  /* 64-bit l2: touch all entries except for PAE compat guests. */
>>  #define SHADOW_FOREACH_L2E(_sl2mfn, _sl2e, _gl2p, _done, _dom, _code)       
>> \
>>  do {                                                                        
>> \
>> -    int _i;                                                                 
>> \
>> -    int _xen = !shadow_mode_external(_dom);                                 
>> \
>> +    unsigned int _i, _end = SHADOW_L2_PAGETABLE_ENTRIES;                    
>> \
>>      shadow_l2e_t *_sp = map_domain_page((_sl2mfn));                         
>> \
>>      ASSERT_VALID_L2(mfn_to_page(_sl2mfn)->u.sh.type);                       
>> \
>> -    for ( _i = 0; _i < SHADOW_L2_PAGETABLE_ENTRIES; _i++ )                  
>> \
>> +    if ( is_pv_32bit_domain(_dom) /* implies !shadow_mode_external(_dom) */ 
>> && \
> 
> As this is a comment, I think can reasonably be
> 
> /* implies !shadow_mode_external */
> 
> which shortens it enough to maintain the RHS justification.

I would certainly have done it without pushing out the escape, but both
alternative variants look worse to me: In

    /* Implies !shadow_mode_external(_dom) */                               \
    if ( is_pv_32bit_domain(_dom) &&                                        \
         mfn_to_page(_sl2mfn)->u.sh.type != SH_type_l2_64_shadow )          \
        _end = COMPAT_L2_PAGETABLE_FIRST_XEN_SLOT(_dom);                    \

it isn't clear that the comment applies to only the first part of the
conditions, whereas
    
    if ( /* Implies !shadow_mode_external(_dom): */                         \
         is_pv_32bit_domain(_dom) &&                                        \
         mfn_to_page(_sl2mfn)->u.sh.type != SH_type_l2_64_shadow )          \
        _end = COMPAT_L2_PAGETABLE_FIRST_XEN_SLOT(_dom);                    \

looks more clumsy to me. I'm not very likely to accept a suggestion to
for the former route; if you think the latter variant is strictly
better than the original, I might make the change while committing.

Hmm, or maybe

    if ( mfn_to_page(_sl2mfn)->u.sh.type != SH_type_l2_64_shadow )          \
         /* Implies !shadow_mode_external(_dom): */                         \
         is_pv_32bit_domain(_dom) &&                                        \
        _end = COMPAT_L2_PAGETABLE_FIRST_XEN_SLOT(_dom);                    \

?

Jan