Re: [Xen-ia64-devel] Protection key support for PV domains

[Isaku Yamahata <yamahata@xxxxxxxxxxxxx>]

On Mon, Jul 23, 2007 at 02:09:02PM +0200, Dietmar Hahn wrote:

> My problem is, I'am still unsure about the usage model.
> With your suggestion (put pkrs in mapped_regs)  the domU can get the pkr's 
> only with reading the memory, but I believe this isn't used often (In our 
> ported OS we never do this). Therewith the win of performance is only 
> marginal.
> The write can be done by writing to pkrs[] and doing a hypercall with the 
> register number.
> The problem with the pkr is the order in which the cpu-prk's are written from 
> the domU. The hypervisor must take care of the order. Within context_switch() 
> the cpu-pkr's must be loaded from the pkrs[] vector. This must be done in the 
> correct order, because writing a pkr-register leads to invalidating already 
> written registers with the same key.
> Imaginable for better performance would be that the domU writes multiple 
> pkr's 
> in memory, maybe invalidating multiple keys, and only one hypercall is used 
> with a bitmask (one bit for every register). But in this case the order gets 
> lost or we need a special interface, where the domU provides this 
> information. But than the hypercall interface is a lot more complex.
> The other way would be the hypervisor doesn't look at the order and the domU 
> must ensure that no key is twice in pkrs[].
> For our port we won't need the order handling in the hypervisor. But I think 
> a 
> correct implementation should be done.
> 
> 1. pkrs[] in struct arch_vcpu:
>    - hypervisor can simple take care about the order
>    - no new hypercall
>    - mov r=pkrs[], mov pkr[]=r and a hyperprivop
>    - simple domU interfaces
> 2. pkrs[] in struct mapped regs:
>    - not in the ABI.
>    - fast read possible (maybe not really needed)
>    - new hypercall for writing cpu-pkr
>    - mov r=pkrs[], mov pkr[]=r, a hyperprivop and the new hypercall
>    1. if only setting one pkr per hypercall than hypervisor can simple take
>       care of order.
>    2. if setting multiple prks[] per hypercall and handling the order in the
>       hypervisor, than more support from domU needed.
> 
> So what says your long time experience?

Thanks for detailed explanation.
Given that the usage model is unsure, I think the simple way is better.
After it is done, then we can re-consider on whether the more complex
way is necessary or not.

thanks,
-- 
yamahata

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