Re: [Xen-devel] [RFC PATCH 1/1] Add pci_hole_min_size

[Don Slutz <dslutz@xxxxxxxxxxx>]

On 03/04/14 08:25, George Dunlap wrote:
> On Fri, Feb 28, 2014 at 8:15 PM, Don Slutz <dslutz@xxxxxxxxxxx> wrote:
> > This allows growing the pci_hole to the size needed.
> You mean, it allows the pci hole size to be specified at boot
Yes.

>   -- the
> pci hole still cannot be enlarged dynamically in hvmloader, correct?
If I am correctly understanding you, this is in reference to:

/*
     * At the moment qemu-xen can't deal with relocated memory regions.
     * It's too close to the release to make a proper fix; for now,
     * only allow the MMIO hole to grow large enough to move guest memory
     * if we're running qemu-traditional.  Items that don't fit will be
     * relocated into the 64-bit address space.   */


so the answer is no, however using pci_hole_min_size can mean that 
allow_memory_relocate is not needed for upstream QEMU.



> What's your intended use case for this?
>
>   -George
If you add enough PCI devices then all mmio may not fit below 4G which may not 
be the layout the user wanted. This allows you to increase the below 4G address 
space that PCI devices can use and therefore in more cases not have any mmio 
that is above 4G.

There are real PCI cards that do not support mmio over 4G, so if you want to 
emulate them precisely, you may also need to increase the space below 4G for 
them.  There are drivers for these cards that also do not work if they have 
their mmio space mapped above 4G.

   -Don Slutz


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