Re: [Xen-ia64-devel] [PATCH] [RFC] [TAKE3] P2M/VP (incomplete) patches

[Isaku Yamahata <yamahata@xxxxxxxxxxxxx>]

Hi. Thanks for your comment.


On Fri, Mar 24, 2006 at 11:11:38PM +0800, Tian, Kevin wrote:

> Maybe you have to make a 
> priority list, and see whether some core components can be split into 
> self-contained parts with major cleanup efforts paid for them first.

O.K. I will do.


> >- grant table API clean up
> >  This is necessary for merging to xen/x86 upstream.
> >  A documentation might be also needed.
> >  - grant table read-only mapping
> 
> Do you mean the grant table itself presents as read-only to guest? X86 
> version awards xenlinux to manage allocation/release of grant table 
> entries.

No. I should have been more concrete.
The current patch ignores GTF_readonly flags.
It should be supported. 


> >- copy_to_guest(), copy_from_guest()
> >  They are broken.
> >  Their copy may success or may result in EFAULT depending on tlb
> >cache state.
> >  Fortunately xen/PPC port already solved similar problems.
> 
> That's a potential issue for a long time, which seldom occurs previously  
> because dom0 memory is contiguous at that time and thus large TLB 
> entry like 16M can be injected into VHPT and machine TLB. However 
> after transited to p2m model with indication for incontiguous memory, 
> people get many smaller TLB entries (16k) and thus copy_to/from_guest 
> is more likely to fail.
> 
> IA64 is a bit different as PPC, since xen/ia64 can walk guest virtual 
> address directly while PPC can't. So normally people have two options:
> 
> - Injected faults into guest when failed, and then let guest re-execute 
> hypercall. Cons is that forward progress may not be ensured when 
> parameter buffer is huge. May need some transient data to track the 
> progress.

A hypercall may require more than one copy_{to, from}_guest().
In such case just re-executing hypercall doesn't work.
It is needed for xen to track hypercall progress and to resume
after injecting faults somehow.


> - Do like xen/PPC way, to pass by machine physical address with 
> scatter/gather list. Cons is that it makes worse when translation for guest 
> buffer exists in mTLB and VHPT. (No poor man there)
> 
> Way to balance.

-- 
yamahata

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