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

[David Hildenbrand]

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.

--
Cheers,

David / dhildenb