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

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Date:	Wed, 15 Oct 2008 09:23:53 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	Chris Lalancette <clalance@...hat.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH]: Fix Xen domU boot with batched	 mprotect

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

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.

>  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.
>   

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

    J
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