lkml.org 
[lkml]   [2008]   [Oct]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Xen-devel] [PATCH]: Fix Xen domU boot with batched mprotect
>>> Chris Lalancette <clalance@redhat.com> 15.10.08 13:03 >>>
>The right thing to do is to use arbitrary_virt_to_machine, so that we can be
>sure we are modifying the right pfn. This unfortunately introduces a
>performance penalty because of a full page-table-walk, but we can avoid that
>penalty for pages in the p2m list by checking if virt_addr_valid is true, and if
>so, just doing the lookup in the p2m table.

Could you explain how virt_addr_valid() can validly be used here? Looking
at its implementation

#define virt_addr_valid(kaddr) pfn_valid(__pa(kaddr) >> PAGE_SHIFT)

a kaddr in kmap space (i.e. above high_memory) would return a bogus
physical address, and hence pfn_valid() on the resulting pfn is meaningless.

I'd instead simply compare the address in question against high_memory,
and perhaps instead of in arbitrary_virt_to_machine() in
ptep_modify_prot_commit() under an #ifdef CONFIG_HIGHPTE. But
performance-wise, CONFIG_HIGHPTE sucks under Xen anyway, so you'd
better not turn this on in the first place. We may want/need to provide
a means to disable this at run time so the same kernel when run natively
could still make use of it, but without impacting performance under Xen.

Jan



\
 
 \ /
  Last update: 2008-10-15 17:27    [W:0.053 / U:0.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site