Re: [Qemu-devel] [RFC PATCH 0/9] hw/acpi: make build_madt arch agnostic

[Wei Yang]

On Fri, Jun 21, 2019 at 10:11:31AM +0200, Igor Mammedov wrote:
>On Fri, 21 Jun 2019 08:56:44 +0800
>Wei Yang <address@hidden> wrote:
>
>> On Thu, Jun 20, 2019 at 05:04:29PM +0200, Igor Mammedov wrote:
>> >On Thu, 20 Jun 2019 14:18:42 +0000
>> >Wei Yang <address@hidden> wrote:
>> >  
>> >> On Wed, Jun 19, 2019 at 11:04:40AM +0200, Igor Mammedov wrote:  
>> >> >On Wed, 19 Jun 2019 14:20:50 +0800
>> >> >Wei Yang <address@hidden> wrote:
>> >> >    
>> >> >> On Tue, Jun 18, 2019 at 05:59:56PM +0200, Igor Mammedov wrote:    
>> >> >> >
>> >> >> >On Mon, 13 May 2019 14:19:04 +0800
>> >> >> >Wei Yang <address@hidden> wrote:
>> >> >> >      
>> >> >> >> Now MADT is highly depend in architecture and machine type and 
>> >> >> >> leaves
>> >> >> >> duplicated code in different architecture. The series here tries to 
>> >> >> >> generalize
>> >> >> >> it.
>> >> >> >> 
>> >> >> >> MADT contains one main table and several sub tables. These sub 
>> >> >> >> tables are
>> >> >> >> highly related to architecture. Here we introduce one method to 
>> >> >> >> make it
>> >> >> >> architecture agnostic.
>> >> >> >> 
>> >> >> >>   * each architecture define its sub-table implementation function 
>> >> >> >> in madt_sub
>> >> >> >>   * introduces struct madt_input to collect sub table information 
>> >> >> >> and pass to
>> >> >> >>     build_madt
>> >> >> >> 
>> >> >> >> By doing so, each architecture could prepare its own sub-table 
>> >> >> >> implementation
>> >> >> >> and madt_input. And keep build_madt architecture agnostic.      
>> >> >> >
>> >> >> >I've skimmed over patches, and to me it looks mostly as code movement
>> >> >> >without apparent benefits and probably a bit more complex than what 
>> >> >> >we have now
>> >> >> >(it might be ok cost if it simplifies MADT support for other boards).
>> >> >> >
>> >> >> >Before I do line by line review could you demonstrate what effect new 
>> >> >> >way
>> >> >> >to build MADT would have on arm/virt and i386/virt (from NEMU). So it 
>> >> >> >would be
>> >> >> >possible to estimate net benefits from new approach?
>> >> >> >(PS: it doesn't have to be patches ready for merging, just a dirty 
>> >> >> >hack
>> >> >> >that would demonstrate adding MADT for new board using mad_sub[])
>> >> >> >      
>> >> >> 
>> >> >> Per APIC spec 5.2.12, MADT contains a *main* table and several *sub* 
>> >> >> tables
>> >> >> (Interrupt Controllere), so the idea is give a callback hook in
>> >> >> AcpiDeviceIfClass for each table, including *main* and *sub* table.
>> >> >> 
>> >> >> Current AcpiDeviceIfClass has one callback pc_madt_cpu_entry for some 
>> >> >> *sub*
>> >> >> tables, after replacing the AcpiDeviceIfClass will look like this:
>> >> >> 
>> >> >> typedef struct AcpiDeviceIfClass {
>> >> >>     /* <private> */
>> >> >>     InterfaceClass parent_class;
>> >> >> 
>> >> >>     /* <public> */
>> >> >>     void (*ospm_status)(AcpiDeviceIf *adev, ACPIOSTInfoList ***list);
>> >> >>     void (*send_event)(AcpiDeviceIf *adev, AcpiEventStatusBits ev);
>> >> >> -   void (*madt_cpu)(AcpiDeviceIf *adev, int uid,
>> >> >> -                    const CPUArchIdList *apic_ids, GArray *entry);
>> >> >> +   madt_operation madt_main;
>> >> >> +   madt_operation *madt_sub;
>> >> >> } AcpiDeviceIfClass;
>> >> >> 
>> >> >> By doing so, each arch could have its own implementation for MADT.
>> >> >> 
>> >> >> After this refactoring, build_madt could be simplified to:
>> >> >> 
>> >> >> build_madt(GArray *table_data, BIOSLinker *linker, PCMachineState 
>> >> >> *pcms,
>> >> >>            struct madt_input *input)
>> >> >> {
>> >> >>     ...
>> >> >> 
>> >> >>     if (adevc->madt_main) {
>> >> >>         adevc->madt_main(table_data, madt);
>> >> >>     }
>> >> >> 
>> >> >>     for (i = 0; ; i++) {
>> >> >>         sub_id = input[i].sub_id;
>> >> >>         if (sub_id == ACPI_APIC_RESERVED) {
>> >> >>             break;
>> >> >>         }
>> >> >>         opaque = input[i].opaque;
>> >> >>         adevc->madt_sub[sub_id](table_data, opaque);
>> >> >>     }
>> >> >> 
>> >> >>     ...
>> >> >> }
>> >> >> 
>> >> >> input is a list of data necessary to build *sub* table. Its details is 
>> >> >> also
>> >> >> arch dependent.    
>> >> >I've got general idea reading patches in this series.
>> >> >As I've mentioned before it's hard to generalize MADT since it
>> >> >mostly contains entries unique for target/board.
>> >> >Goal here isn't generalizing at any cost, but rather find out
>> >> >if there is enough common code to justify generalization
>> >> >and if it allows us to reduce code duplication and simplify.
>> >> >    
>> >> >> For following new arch, what it need to do is prepare the input array 
>> >> >> and
>> >> >> implement necessary *main*/*sub* table callbacks.    
>> >> >What I'd like to see is the actual patch that does this,
>> >> >to see if it has any merit and to compare to the current
>> >> >approach.    
>> >> 
>> >> I didn't get some idea about your approach. Would you mind sharing more 
>> >> light?  
>> >With current approach, 'each board' has its own MADT build routine.
>> >Considering that there is very little to share between different
>> >implementations it might be ok.
>> >
>> >This series just add extra data structure for board to populate
>> >and a bunch of callbacks for every record type. Essentially all
>> >the code we have now is still there. It was just moved elsewhere
>> >and made available via callbacks.  
>> 
>> Yes, you are right.
>> 
>> >This series touches only pc/q35 machines and it's not apparent
>> >to me why it's any better than what we have now.  
>> 
>> This is the demo for i386. In case you think this approach is reasonable, it
>> could be applied to arm. And then for new board, we can apply the same
>> approach.
>well, it's not obvious from i386 demo, how it's any better than what
>we have now. It lacks arm/virt patches so we could see if it would make
>anything better or not.
>

ok, let me add arm/vrit part.

>If I were to talk about i386 demo alone, then I'd say it just makes
>code more complex and I'd leave existing MADT code as it.

-- 
Wei Yang
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