Re: [PATCH v5 4/5] i386/pc: relocate 4g start to 1T where applicable

[Joao Martins]

On 6/17/22 13:32, Igor Mammedov wrote:
> On Fri, 17 Jun 2022 13:18:38 +0100
> Joao Martins <joao.m.martins@oracle.com> wrote:
>> On 6/16/22 15:23, Igor Mammedov wrote:
>>> On Fri, 20 May 2022 11:45:31 +0100
>>> Joao Martins <joao.m.martins@oracle.com> wrote:
>>>> +                                hwaddr above_4g_mem_start,
>>>> +                                uint64_t pci_hole64_size)
>>>> +{
>>>> +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
>>>> +    X86MachineState *x86ms = X86_MACHINE(pcms);
>>>> +    MachineState *machine = MACHINE(pcms);
>>>> +    ram_addr_t device_mem_size = 0;
>>>> +    hwaddr base;
>>>> +
>>>> +    if (!x86ms->above_4g_mem_size) {
>>>> +       /*
>>>> +        * 32-bit pci hole goes from
>>>> +        * end-of-low-ram (@below_4g_mem_size) to IOAPIC.
>>>> +        */
>>>> +        return IO_APIC_DEFAULT_ADDRESS - 1;  
>>>
>>> lack of above_4g_mem, doesn't mean absence of device_mem_size or anything 
>>> else
>>> that's located above it.
>>>   
>>
>> True. But the intent is to fix 32-bit boundaries as one of the qtests was 
>> failing
>> otherwise. We won't hit the 1T hole, hence a nop.
> 
> I don't get the reasoning, can you clarify it pls?
> 

I was trying to say that what lead me here was a couple of qtests failures 
(from v3->v4).

I was doing this before based on pci_hole64. phys-bits=32 was for example one
of the test failures, and pci-hole64 sits above what 32-bit can reference.

>>  Unless we plan on using
>> pc_max_used_gpa() for something else other than this.
> 
> Even if '!above_4g_mem_sizem', we can still have hotpluggable memory region
> present and that can  hit 1Tb. The same goes for pci64_hole if it's configured
> large enough on CLI.
> 
So hotpluggable memory seems to assume it sits above 4g mem.

pci_hole64 likewise as it uses similar computations as hotplug.

Unless I am misunderstanding something here.

> Looks like guesstimate we could use is taking pci64_hole_end as max used GPA
> 
I think this was what I had before (v3[0]) and did not work.

Let me revisit this edge case again.

[0] https://lore.kernel.org/all/20220223184455.9057-5-joao.m.martins@oracle.com/