Re: [patch 0/3] RFC: provide synchronous registers in kvm_run

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 22/12/11 13:54, Avi Kivity wrote:
>> My main concern was the prefix register (this is a per cpu register that
>> defines the address of two pages that are swapped with the pages at 0 for this cpu).
>> SMP on s390 is done that way (e.g. interrupt things are stored in page 0 for this cpu)
>> The storage that qemu sees is storage without prefix. For architecture compliance
>> we actually must check _every_ memory access if it hits the prefix/swpa area and 
>> the add/subtract the prefix value. 
> 
> Those are only memory accesses coming from the cpu, yes?  Why does
> userspace have to access them at all?  I imagine DMA ignores it
> completely since it doesn't come from the cpu.

Not sure if I got you question...(just ask again if that doesnt aswer it)

The prefix page contains HW-defined content (like the PSWs for the different
interrupt types) as well as some OS-defined values (for CPU local data structures
as well as a place to store information in critical sections) 
The prefix page (and the swap area) must not be used for device I/O (since it will
be broken as you pointed out), but some I/O instructions can and will write status
information to the prefix page. For example the channel subsystem driver in Linux
will use an area in the prefix page as a store address for some instructions.

So let me phrase the above sentence differently:
For architecture compliance we actually must check every memory access that is done
on behalf of a guest cpu and was not already handled by the host kernel.

PS: Most of the things are really handled in the kernel. As you can see, the current
paravirtual I/O stack does not need the prefix at all