Re: [PATCH 08/22] x86/pv: rewrite how building PV dom0 handles domheap mappings

["El Yandouzi, Elias" <eliasely@amazon.com>]

Hi Jan,

I have been looking at this series recently and tried my best
to address your comments. I'll shortly to the other patches too.

On 22/12/2022 11:48, Jan Beulich wrote:
> On 16.12.2022 12:48, Julien Grall wrote:
>> From: Hongyan Xia <hongyxia@amazon.com>
>>
>> Building a PV dom0 is allocating from the domheap but uses it like the
>> xenheap. This is clearly wrong. Fix.
> 
> "Clearly wrong" would mean there's a bug here, at lest under certain
> conditions. But there isn't: Even on huge systems, due to running on
> idle page tables, all memory is mapped at present.

I agree with you, I'll rephrase the commit message.

> 
>> @@ -711,22 +715,32 @@ int __init dom0_construct_pv(struct domain *d,
>>           v->arch.pv.event_callback_cs    = FLAT_COMPAT_KERNEL_CS;
>>       }
>>   
>> +#define UNMAP_MAP_AND_ADVANCE(mfn_var, virt_var, maddr) \
>> +do {                                                    \
>> +    UNMAP_DOMAIN_PAGE(virt_var);                        \
> 
> Not much point using the macro when ...
> 
>> +    mfn_var = maddr_to_mfn(maddr);                      \
>> +    maddr += PAGE_SIZE;                                 \
>> +    virt_var = map_domain_page(mfn_var);                \
> 
> ... the variable gets reset again to non-NULL unconditionally right
> away.

Sure, I'll change that.

> 
>> +} while ( false )
> 
> This being a local macro and all use sites passing mpt_alloc as the
> last argument, I think that parameter wants dropping, which would
> improve readability.

I have to disagree. It wouldn't improve readability but make only make 
things more obscure. I'll keep the macro as is.

> 
>> @@ -792,9 +808,9 @@ int __init dom0_construct_pv(struct domain *d,
>>               if ( !l3e_get_intpte(*l3tab) )
>>               {
>>                   maddr_to_page(mpt_alloc)->u.inuse.type_info = PGT_l2_page_table;
>> -                l2tab = __va(mpt_alloc); mpt_alloc += PAGE_SIZE;
>> -                clear_page(l2tab);
>> -                *l3tab = l3e_from_paddr(__pa(l2tab), L3_PROT);
>> +                UNMAP_MAP_AND_ADVANCE(l2start_mfn, l2start, mpt_alloc);
>> +                clear_page(l2start);
>> +                *l3tab = l3e_from_mfn(l2start_mfn, L3_PROT);
> 
> The l2start you map on the last iteration here can be re-used ...
> 
>> @@ -805,9 +821,17 @@ int __init dom0_construct_pv(struct domain *d,
>>           unmap_domain_page(l2t);
>>       }
> 
> ... in the code the tail of which is visible here, eliminating a
> redundant map/unmap pair.

Good catch, I'll remove the redundant pair.

> 
>> @@ -977,8 +1001,12 @@ int __init dom0_construct_pv(struct domain *d,
>>        * !CONFIG_VIDEO case so the logic here can be simplified.
>>        */
>>       if ( pv_shim )
>> +    {
>> +        l4start = map_domain_page(l4start_mfn);
>>           pv_shim_setup_dom(d, l4start, v_start, vxenstore_start, vconsole_start,
>>                             vphysmap_start, si);
>> +        UNMAP_DOMAIN_PAGE(l4start);
>> +    }
> 
> The, at the first glance, redundant re-mapping of the L4 table here could
> do with explaining in the description. However, I further wonder in how
> far in shim mode eliminating the direct map is actually useful. Which is
> to say that I question the need for this change in the first place. Or
> wait - isn't this (unlike the rest of this patch) actually a bug fix? At
> this point we're on the domain's page tables, which may not cover the
> page the L4 is allocated at (if a truly huge shim was configured). So I
> guess the change is needed but wants breaking out, allowing to at least
> consider whether to backport it.
> 

I will create a separate patch for this change.

> Jan
>