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

[Christian KÃnig]

Am 12.12.2017 um 19:12 schrieb Bjorn Helgaas:
> [+cc Boris, Juergen, xen-devel]
>
> On Mon, Dec 11, 2017 at 04:04:52PM +0100, Christian KÃnig 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?

256GB was just an arbitrary number I've thought should work for at least 
my use case.

And yes Boris is working on a more complete and cleaner fix. I agree 
that we can completely drop my patch for now.

> 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.

That patch looks more like it only applies to Xen guests, but not dom0. 
But take that with a grain of salt I really don't know anything about 
that code.

Christian.

> > Signed-off-by: Christian KÃnig <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