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Re: [Xen-devel] BUG: bad page map under Xen

On 23/10/13 16:36, Konrad Rzeszutek Wilk wrote:
On Mon, Oct 21, 2013 at 04:12:56PM +0100, Jan Beulich wrote:
On 21.10.13 at 16:44, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
On Mon, Oct 21, 2013 at 03:27:50PM +0100, Jan Beulich wrote:
On 21.10.13 at 16:18, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
On Mon, Oct 21, 2013 at 04:06:07PM +0200, Lukas Hejtmanek wrote:
         Region 2: Memory at 380fff000000 (64-bit, prefetchable) [size=8M]
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -92,6 +92,9 @@ static void __init xen_add_extra_mem(u64 start, u64 size)

                 __set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
+       /* Anything past the balloon area is marked as identity. */
+       for (pfn = xen_max_p2m_pfn; pfn < MAX_DOMAIN_PAGES; pfn++)
+               __set_phys_to_machine(pfn, IDENTITY_FRAME(pfn));

Hardly - MAX_DOMAIN_PAGES derives from
CONFIG_XEN_MAX_DOMAIN_MEMORY, which in turn is unrelated
to where MMIO might be. Should you perhaps simply start from

Looks like your mailer ate some words.

I don't think so - they're all there in the text you quoted.

an all 1:1 mapping, inserting the RAM translations as you find

Yeah, as this code can be called for the regions under 4GB. Definitly
needs more analysis.

Were you suggesting a lookup when we scan the PCI devices? (xen_add_device)?

That was for PVH, and is obviously fragile, as there can be MMIO
regions not matched by any PCI device's BAR. We could hope for
all of them to be below 4Gb, but I think (based on logs I got to see
recently from a certain vendor's upcoming systems) this isn't going
to work out.

This is the patch I had in mind that I think will fix these issues. But
I would appreciate testing it and naturally send me the dmesg if possible.

I think there is a simpler way to handle this.

If INVALID_P2M_ENTRY implies 1:1 and we arrange:

a) pfn_to_mfn() to return pfn if the mfn is missing in the p2m
b) mfn_to_pfn() to return mfn if p2m(m2p(mfn)) != mfn and there is no m2p override.


a) The identity p2m entries can be removed.
b) _PAGE_IOMAP becomes unnecessary.


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