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

From: Xu, Anthony <>
Date: 2008-01-22 17:56:26
Xu, Anthony wrote:
> 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.

We will use per-cpu ia64_max_tr_num.

>> 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_ptr_entry.

One concern about this
Where can we put Macro #define KVM_ADDRESS 0xD000..?
It's not suitable to put it in file tlb.c.

>> 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
> free.
> 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. Agree. 

More thinking.
If we put kvm_ into the names of the exported interfaces, then it is the
kvm specific interfaces,
Seems it is not appropriate to put the interfaces in tlb.c.

Basically this patch provides common TR insert/purge interface for all
modules, not specific to KVM.
Currently only KVM/IA64 need these interfaces, maybe later on other
modules also need it.
What do you think?

- Anthony

> Will send out patch soon per your comments.
> Thanks
> - Anthony
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Received on Tue Jan 22 17:56:58 2008

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