RE: [PATCH]Add TR insert/purge interface for add-on component

From: Xu, Anthony <>
Date: 2008-01-22 17:35:31
Thanks for your comments

Luck, Tony wrote:
> Looks pretty good.
> Stylistically it would be nicer to initialize ia64_max_tr_num to 8
> (with 
> a comment that this is the least smallest allowed value allowed by the
> architecture - SDM p.2:44 section and increase this if
> PAL_VM_SUMMARY indicates that the current processor model supports a
> larger value.  That 
> way you are sure that it never has a larger value that it should. N.B.
> SGI ship systems with mixed processor models, so to be completely
> correct 
> here you either need ia64_max_tr_num to be a per-cpu value, or to
> make sure 
> you only set it to the smallest value supported by any cpu on the
> system. 
Agree, we can initialize ia64_max_tr_num to 8.

> Your overlap checking code only looks like it checks for overlaps
> among 
> the new entries being inserted via this interface.  Is there some
> other non-obvious way that these are prevented from overlapping with
> the TR entries 
> in use by the base kernel (ITR[0]+DTR[0] mapping the kernel in region
> 5, 
> ITR[1] for the PAL code and DTR[1] for the current kernel stack
> granule? 
> I don't know how kvm will use this interface, perhaps the virtual
> address 
> range is limited to areas that can't overlap?  If so, perhaps the
> ia64_itr_enty() routine should check that the va,size arguments are
> in the virtual address 
> range that KVM will use?
Kvm only use TRs whose virtual address starts with 0xD000..
I will add virtual address check for va at function ia64_itr_entry and

> ia64_itr_entry() should check that 'log_size' is a supported page
> size for this processor, and that 'va' is suitably aligned for that
> size. 
Agree, we need check log_size in ia64_itr_entry and ia64_ptr_entry.
Va doesn't need to be aligned for that size, if you look at itr spec.
We can make it aligned for that size.

> ia64_ptr_entry() perhaps this should just take a 'target_mask' and
> 'reg' argument.  Then it could skip all the overlap checks and just
> lookup 
> the address+size in the __per_cpu_idtrs[][][] array return an error if
> you try to purge something that you didn't set up (->pte == 0).
> Calling 
> this routine 'ia64_purge_tr' (which is the name in the header comment
> :-) 
> would help note the non-symmetric calling arguments between insert
> and purge. 
Ok, we will do it.

> What is the expected usage pattern for itr, dtr, itr+dtr mappings by
> KVM? 
KVM/IA64 use two itrs and two dtrs.

> If you are going to allocate enough that there might be contention, it
> could be better to start the allocation search for i+d entries at the
> top 
> and work downwards, while allocating just-i and just-d entries from
> low 
> numbers working up.  That might avoid issues with not having an i+d
> pair available becaue all the odd entries were allocated for 'i' and
> all the 
> even ones allocated for 'd' ... so even though there are plenty of
> free 
> TR registers, none of the free ones are in pairs.
The __per_cpu_idtrs is to reflect the machine ITRs and DTRS.
And ITR and DTR are separate.
It is impossible that odd entries are for 'I' and even ones for 'd'.
In theory, i+d pair can not be available even if there are plenty of
While KVM/IA64 only uses two pair, that will not happen.
If we want to provide a general TR insert/purge interfaces, we need to
handle this issue.
One possible solution is we don't support i+d pair allocation

> Maybe we should put a 'kvm_' into the names of the exported
> interfaces? 
> Sadly there isn't a way to just make these visible to KVM ... but I'd
> like 
> to make it crystal clear  that other drivers should not use these.

Will send out patch soon per your comments.

- Anthony

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Received on Tue Jan 22 17:36:21 2008

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