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Re: [Xen-devel] [PATCH]: Fix Xen domU boot with batched mprotect

Jan Beulich wrote:
Chris Lalancette <clalance@xxxxxxxxxx> 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.

virt_addr_valid() is supposed to be usable in this circumstace. The comment says "virt_to_page(kaddr) returns a valid pointer if and only if virt_addr_valid(kaddr) returns true", which implies that virt_addr_valid() returns a meaningful result on all addresses - and if not, it should be fixed.

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.

I suppose, but I don't think there's much cost in making it generally robust.

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.

That's a secondary issue. What's the source of the performance hit? Just all the extra kmap_atomic operations?


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