Re: [PATCH] mem/x86: add processor address space check for VM memory

[David Hildenbrand <david@redhat.com>]

On 18.09.23 12:54, Ani Sinha wrote:
> On Mon, Sep 18, 2023 at 3:49 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 18.09.23 12:11, Ani Sinha wrote:
>>
>>>
>>>      Ok hopefully my last question. I am still confused on something.
>>>      Does the above mean that the hole64 will actually start from an
>>>      address that is beyond maxram? Like basically if you added all of
>>>      ram_below_4G, ram_above_4G, hot plug_mem and pci_hole64 then can it
>>>      exceed maxram? I think it will. Does this not an issue?
>>
>> If you'd have a 2 GiB VM, the device memory region and hole64 would
>> always be placed >= 4 GiB address, yes.
>>
>> As maxram is just a size, and not a PFN, I don't think there is any
>> issue with that.
> 
> So this is all just a scheme to decide what to place where with maxram
> amount of memory available. When the processor needs to access the

Yes. ram_size and maxram_size are only used to create the memory layout.

> memory mapped PCI device, its simply dynamically mapped to the
> available physical ram. Is my understanding correct here?

I'm no expert on that, but from my understanding that's what the 
pci/pci64 hole is for -- mapping PCI BARs into these areas, such that 
they don't conflict with actual guest RAM. That's why we still account 
these "holes" as valid GFN that could be used+accessed by the VM once a 
PCI BAR gets mapped in there.

-- 
Cheers,

David / dhildenb