RE: [Xen-ia64-devel] flush.S not para-virtualized

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

Hmmm... Kevin, you may be right.  My memory is dim on this
(as it was >15 months ago), but at some point an fc instruction
in linux did cause an unexpected trap.  This was back when
I was still working with a privified kernel, probably older
than 2.6.9.  IIRC, the problem occurred during one of the
patches in linux/arch/ia64/kernel/patch.c.

It's possible that this was due to a bug, or it may have
been dealing with an epc page.

Dan

> -----Original Message-----
> From: Tian, Kevin [mailto:kevin.tian@xxxxxxxxx] 
> Sent: Monday, March 27, 2006 9:16 AM
> To: Tristan Gingold; Magenheimer, Dan (HP Labs Fort Collins); 
> xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-ia64-devel] flush.S not para-virtualized
> 
> >From: Tristan Gingold [mailto:Tristan.Gingold@xxxxxxxx]
> >Sent: 2006å3æ27æ 23:52
> >
> >Le Lundi 27 Mars 2006 17:24, Magenheimer, Dan (HP Labs Fort Collins)
> >a Ãcrit :
> >> Agreed, this needs to be paravirtualized.
> >So, everybody agree.
> >I will add a fc.i hyperprivop.
> >
> >However, I fear the hyperprivop-ized version of flush.S would be very
> >slow.
> >Should we also create an hyperprivop for something like
> >flush_icache_range ?
> >
> >Tristan.
> 
> Normally more thinking brings more questions. :-) SDM says:
> 
> When executed at privilege level 0, fc and fc.i perform no 
> access rights or 
> protection key checks. At other privilege levels, fc and fc.i 
> perform access 
> rights checks as if they were 1-byte reads, but do not perform any 
> protection key checks (regardless of PSR.pk).
> 
> Easy to see hint here to protect memory pages of higher 
> region can't be 
> affected by lower privilege level, or else the performance 
> may be affected 
> a lot by malicious programs. Then let's see which cases 1-byte reads 
> can't pass access rights checks in current environment:
> 
> First we have TLB.pl == 2, and xenlinux kernel also executes 
> at cpl==2. In 
> all 8 types of access rights, 0-6 all support read at same 
> privilege level 
> with only exception as type 7 (execute,promote/read,execute). However 
> that page is special to contain 'epc' instruction. Content on 
> that page is 
> normally fixed and stable, and it's difficult to find good 
> reason to flush 
> cache entry for that page.
> 
> If it's true with only one weak exception, is it really 
> worthy of virtualizing 
> fc.i? :-)
> 
> Thanks,
> Kevin
> 

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