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

Konrad Rzeszutek Wilk
2013-10-25T23:39:34Z

David Vrabel <dvrabel@cantab.net> wrote:
>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.

You are suggesting not touching the p2m code and instead change the the xen-make-pte to assume that INVALID and IDENTITY should have the same behaviour. Interesting.
>
>
>> 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@citrix.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__ */

You are welcome to suggest it to hpa. Last time a similar patch was proposed he was not too thrilled about it.

Looking forward to your p2m code surgery!