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[Xen-devel] auto_translated_physmap in Fedora

To: <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] auto_translated_physmap in Fedora
From: Pete Zaitcev <zaitcev@xxxxxxxxxx>
Date: Thu, 6 Jul 2006 19:07:38 -0700
Cc: zaitcev@xxxxxxxxxx, quintela@xxxxxxxxxx
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I noticed one curious thing while working on some issues with Fedora.
At least in 2.6.17-1.2302_FC6xen, the following evaluates to false:

        if (xen_feature(XENFEAT_auto_translated_physmap))
                return pfn;

But the head-xen.S looks like this:

.section __xen_guest
        .ascii  "GUEST_OS=linux,GUEST_VER=2.6"
        .ascii  ",XEN_VER=xen-3.0"
        .ascii  ",VIRT_BASE=0x"; utoh __START_KERNEL_map
        .ascii  ",HYPERCALL_PAGE=0x"; utoh (phys_hypercall_page >> PAGE_SHIFT)
        .ascii  ",FEATURES=writable_page_tables"
        .ascii           "|writable_descriptor_tables"
        .ascii           "|auto_translated_physmap"
        .ascii           "|supervisor_mode_kernel"
        .ascii  ",LOADER=generic"
        .byte   0

Everything continues to work just fine, thanks to runtime checks
of xen_feature() like the above.

So I am curious, if this is something anyone cares about.
Also, why does it happen? If it's a feature which cannot be enabled
(anymore), perhaps we can simply the code and get some performance
by removing the run-time checks.

-- Pete

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