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

[Ani Sinha]

On Mon, Sep 18, 2023 at 4:32 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 18.09.23 13:00, Ani Sinha wrote:
> > On Mon, Sep 18, 2023 at 4:28 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> 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.
> >
> > Yes that was my understanding too but since device drivers need to
> > access those BAR addresses, they need to be mapped to the actual
> > available physical ram.
>
> These devices supply their own RAM memory regions for BARs, that will
> get mapped into the holes. They are not using any boot/hotplug memory.
> It's confusing.

yeah exactly. The actual physical memory is in the device in the form
of BAR registers.