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: 2006年3月27日 23:52
> >
> >Le Lundi 27 Mars 2006 17:24, Magenheimer, Dan (HP Labs Fort Collins)
> >a écrit :
> >> 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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