Re: [Xen-devel] [PATCH v1] Set 1-1 P2M for PCI BARs and MCFG regions- if needed.

From: David Vrabel
Date: Fri Oct 25 2013 - 18:59:06 EST

On 25/10/13 16:03, Konrad Rzeszutek Wilk wrote:

2). Assume that INVALID_P2M_ENTRY is considered 1-1. That would require
auditing of the code and also making sure that any xen_privcmd_mmap
calls use the m2p_add_override.

I'm not sure I understand the concern about the privcmd driver's foreign mapping ioctls (MMAPBATCH_V2 and friends) as these all deal with MFNs directly and don't use the p2m.

However there are some
valid cases in which we need to check for INVALID_P2M_ENTRY -
balloon driver - that might be relaxed.

Yes. And this behaviour will be retained. I think we only need to do the INVALID_P2M_ENTRY => 1:1 when constructing PTEs so all other callers will see INVALID_P2M_ENTRY as before.

However again the P2M code
and the ABI it places are not for the faint of the heart.

I have no fear.

If the above suggestion does work out it will reduce the amount of p2m code /and/ remove a Xen-specific PTE bit. So I think it is worth pursuing.

In the meantime, the following minimal patch is a likely workaround.


x86/mm: set _PAGE_IOMAP in io_remap_pfn_range().

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
arch/x86/include/asm/pgtable.h | 3 +++
1 file changed, 3 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 3d19994..d59b22a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -868,6 +868,9 @@ static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);

+#define io_remap_pfn_range(vma, addr, pfn, size, prot) \
+ remap_pfn_range(vma, addr, pfn, size, (prot) | _PAGE_IOMAP)
#include <asm-generic/pgtable.h>
#endif /* __ASSEMBLY__ */


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