On 12/17/2014 07:33 AM, Gerd Hoffmann wrote:
On Di, 2014-12-16 at 15:01 +0000, Tvrtko Ursulin wrote:
Hi,
On 12/16/2014 02:41 PM, Gerd Hoffmann wrote:
I didn't figure out how each domain knowns which fences to use? They
know how many, but which ones?
I think the guest doesn't have to know because mmio access is trapped by
the hypervisor anyway.
Okay.. so the point is to ensure no domain can hog all fences, but if
the hypervisor is already tracking everything why is this limiting
required then in the guest? Hypervisor could just say all fences are
allocated since it knows how much it gave out per guest.
There is no dynamic allocation. Each guest gets a fixed number of
fences assigned which it can use, pretty much like memory where each
guest gets a fixed piece too.
It's not possible to allow guests direct access to the fence registers
though. And if every fence register access traps into the hypervisor
anyway the hypervisor can easily map the guest virtual fence to host
physical fence, so there is no need to tell the guest which fences it
owns, the number of fences is enough.
That exactly is the part I don't understand - if it is not required to
tell the guest which fences it owns, why it is required to say how many?
With this scheme it could be one guest wants more and can't get them
even if no one else is using any. If not limited they could be
dynamically allocated by the hypervisor, no?
Regards,
Tvrtko
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