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Re: [Xen-devel] Little help with Seabios PV-Drivers for XEN

On Fri, Mar 9, 2012 at 8:59 AM, James Harper
<james.harper@xxxxxxxxxxxxxxxx> wrote:
>> Hello All,
>> I have a little setback with the development of PV Drivers for Xen in 
>> SeaBIOS.
>> The initialization code that runs in 32 Bit is working properly.
>> But, when the system tries to read on the disk I use the ring macros to get a
>> request. The macro usage looks like this:
>> struct blkif_ring * shared = memalign_low(4096,4096); //return
>> 0x000fd630 this above 16bit address space SHARED_RING_INIT(shared); So
>> far I have a pointer located at 0x0009a000 Under 32bit the struct is correct
>> and all is working according to plan.
>> But on 16bit operation read on disk I have struct blkfront_info * shared_ring
>> = container_of(op->drive_g.info->shared)); // I get d630 I should get it from
>> the correct segment, but how?
>> RING_GET_REQUEST(shared_ring); //this returns 0xffff and should be
>> something 0xa010 segment SS or something like that
> Just curios, does 16 bit include the required atomic instructions to 
> manipulate the 32 bit ring counters? Or are you manipulating the ring in 32 
> bit mode and only accessing the requests already retrieved from the ring in 
> 16 bit mode?\
I think that is the problem, the operations inside the macro do not
take into account that they are being executed in a 16bit environment.
>> SeaBios has some macros that convert a pointer in 32Bit to 16Bit by changing
>> the segment register, yet I do not know in what segment the ring is located,
>> and the macros are not applied inside the procedure of the macro, for
>> example:
>> But this will change a 16Bit pointer of segment SS to a 32 bit segment. There
>> is also the reverse but, again I do not know the segment in which I should
>> look for. Lastly the process inside the macro does not get this benefin, and 
>> I
>> do not know if the macro will work with a pointer of size 16bits.
>> Any help will be GREATLY appreciated, I am almost done.
> I'm not sure if this would be useful, but is there a way to hand over the 
> ring to the OS PV drivers and avoid the teardown/setup?
I am not sure that I undestand you, all this is exectuted when the
domain builder is deploying the hvmloader. The next step once the
PV-Drivers work is the shutdown sequence to and proper preparation for
the OS "arrival" (meaning boot seq).
> James

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