Re: [Xen-devel] [PATCH] x86/PCI: limit the size of the 64bit BAR to 256GB

[Bjorn Helgaas <helgaas@xxxxxxxxxx>]

[+cc Boris, Juergen, xen-devel]

On Mon, Dec 11, 2017 at 04:04:52PM +0100, Christian König wrote:
> Xen hides a bit of system memory from the OS for its own purpose by
> intercepting e820. This memory is unfortunately not reported as
> reserved, but rather completely invisible.
> 
> Avoid this address space collision and possible similar problems by
> limiting the size of the newly allocated root hub window to 256GB which
> should be sufficient for the short term.

It sounds like Boris is working on a more complete fix, so I'm going
to drop this for now.  This changelog includes a few more details, but
I think it makes implicit assumptions about where memory and holes
are and how big they are, and it's still not clear why 256GB is the
right number.  Is it connected to the expected size of the BAR, or
related somehow to the size of the hole?

If we need this as a short-term workaround, we can do that, but I
would like to include a reference to f5775e0b6116 ("x86/xen: discard
RAM regions above the maximum reservation") and somehow make what's
going on here a little more explicit.

> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
>  arch/x86/pci/fixup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/pci/fixup.c b/arch/x86/pci/fixup.c
> index 8f86060f5cf6..ed8bc6ab0573 100644
> --- a/arch/x86/pci/fixup.c
> +++ b/arch/x86/pci/fixup.c
> @@ -702,7 +702,7 @@ static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
>       res->name = "PCI Bus 0000:00";
>       res->flags = IORESOURCE_PREFETCH | IORESOURCE_MEM |
>               IORESOURCE_MEM_64 | IORESOURCE_WINDOW;
> -     res->start = 0x100000000ull;
> +     res->start = 0xbd00000000ull;
>       res->end = 0xfd00000000ull - 1;
>  
>       /* Just grab the free area behind system memory for this */
> -- 
> 2.11.0
> 

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