Re: [Xen-devel] C Macros and Xen RING Macros Questions

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Apr 06, 2012 at 07:20:13PM +0900, Daniel Castro wrote:
> hello All,
> 
> I am still working on the PV Drivers for SeaBIOS using upstream qemu.
> And, I have two questions.
> 1.
> Here is the location of all relevant data structs:
> blkfront_info:0x000fd620 shared_ring:0x0009a000 private_ring:0x0009b000

I surmise this is the guest PFNs.

> DEBUG Read op private ring at 0x0009b000-0x000ab000, idx 63478
> Here is my problem, when I do:
>         ring_req =
> GLOBALFLAT2GLOBAL(RING_GET_REQUEST(GET_GLOBALFLAT(bi->private),GLOBALFLAT2GLOBAL(GET_GLOBALFLAT(bi->private)->req_prod_pvt)));
>  //please ignore the MACROS for now, or read further down.
> I get:
> After RING_GET_REQUEST operation ring request is at 0xe18ea40f id:0
> But I have the feeling that the request should be between
> 0x0009b000-0x000ab000. Right?

That would imply a physical address - but if you are running with
pagetables (which I think you are?) then it would be virtual address.

You should have some basic macros to translate from virtual to physical
and vice-versa.

> 
> 2.
> As you can see in the above code I use some SeaBIOS macros to access
> 32Bit addresses in 16Bit code. My second questions is: How the memory

16-bit code. Refresh my memory - does that mean you can only
use segments and no paging? So you need to move a 16-bit window
around to get to your 32-bit address?

> access macros affect the RING macros? Do I need to rewrite the ring
> macros to use the memory macros inside, for example:

It should fit within a 16-bit (64K) contingous memory space.
I don't think it matters where that the block is - as long as you
have a segment selector activated for that 16-bit block.

> /* How big is this ring? */
> #define RING_SIZE(_r)                                                   \
>     ((_r)->nr_ents)

That tells you just how many entries are on it. Not how big
the structure is. For that it is sizeof(struct ..).

> 
> Should be instead:
> /* How big is this ring? */
> #define RING_SIZE(_r)                                                   \
>     (GET_GLOBAL((_r)->nr_ents))
> 
> SeaBIOS macros need to be around ALL memory accesses.
> 
> This is a short message for something that might be to complex to
> explain briefly, so please ask any questions that you deem necessary
> to understand. Right now, I am developing the first stage of boot when
> the BIOS requests address 7c00 to get the Boot sector. Once I get this
> working we should have a working prototype for PV-drivers in seabios.
> 
> Thank you all for your interest,
> 
> Daniel
> 
> -- 
> +-=====---------------------------+
> | +---------------------------------+ | This space intentionally blank
> for notetaking.
> | |   | Daniel Castro,                |
> | |   | Consultant/Programmer.|
> | |   | U Andes                         |
> +-------------------------------------+
> 
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