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RE: [Xen-ia64-devel] Floating-point software assist (FPSWA) on Xen-ia64

To: "Yang, Fred" <fred.yang@xxxxxxxxx>, <xen-ia64-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-ia64-devel] Floating-point software assist (FPSWA) on Xen-ia64
From: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Date: Sun, 29 Jan 2006 16:36:46 -0800
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Thread-topic: [Xen-ia64-devel] Floating-point software assist (FPSWA) on Xen-ia64
> Subject: RE: [Xen-ia64-devel] Floating-point software assist 
> (FPSWA) on Xen-ia64
> My speculation calling FPSWA within Xen will lead to 
> 1. Set FPSR accordingly to Domain kernel with Xen, and then 
> Save and restore domain FP registers before/after calling FPSWA in Xen
> 2. ready to handle FP exceptions caused by FPSWA execution.  
> Note Domain application can't be trusted so it can likely 
> incure FP exception.  Xen FP exception must also able to 
> distinguish exception source in order to unwind stack to 
> deliver to domain handler.   Will one domain impact Xen stability?
> 3. If domains doesn't got passed with FPSWA info, can its FP 
> handler able to handle exception delivered from Xen?
> 4. FP opcode fetch must be performed within Xen (likely 
> within FPSWA) to get from domain application base on IIP.  
> TLB miss may likely happen and Xen can't handle it.
> 5. Domain FPSWA usage time is charged to Xen in stead of itself
> Xen may not able to handle above issues cleanly :-)

You may be right, but it seems like the exact code that Linux
uses should work, except that where Linux passes a signal to
the application, Xen would instead reflect the fp assist
fault.  If I have a chance, I may prototype this just to
see if it works... FPSWA isn't very high priority, but it
would be nice to pass LTP if it is easy to fix.
> To minic native behavior, can we provide a single physical 
> copy of FPSWA for whole system without coping for each new 
> domain created?  That is, each domain will be given extra, 
> other than domain memory, physical block that contains the 
> same FPSWA.  The memory is then exported to domains through 
> EFI descriptor or ELILO (Whatever the native method is) with 
> guest physical.  For P2M, domains will map the FPSWA using 
> exported guest physical and then Xen will instead map with 
> the same machine physical block for all the domains.
> The assumption here is domains won't write to FPSWA to 
> clobber it otherwise Xen has to change mapping attribute.  
> Domain destroy may need to perform special handling on this 
> machine block, but it should be easy once we have page 
> reference count implemented.

This can probably be done also but should wait until after
the physical/machine memory work is redone as otherwise it
might have to be done twice.

If the easy solution (above) doesn't work, we should probably
put it off until later.


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