linux-kernel - Re: [Xen-devel] [PATCH v1] Set 1-1 P2M for PCI BARs and MCFG regions

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Date:	Fri, 25 Oct 2013 23:58:34 +0100
From:	David Vrabel <dvrabel@...tab.net>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	Santosh.Jodh@...rix.com, JBeulich@...e.com,
	boris.ostrovsky@...cle.com, david.vrabel@...rix.com,
	mukesh.rathor@...cle.com, xhejtman@....muni.cz,
	yuval.shaia@...cle.com
Subject: Re: [Xen-devel] [PATCH v1] Set 1-1 P2M for PCI BARs and MCFG regions
 - if needed.

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.

David

8<-------------------------------
x86/mm: set _PAGE_IOMAP in io_remap_pfn_range().

Signed-off-by: David Vrabel <david.vrabel@...rix.com>
---
  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__ */

-- 
1.7.10.4

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