Re: [Xen-devel][RFC] Dynamic modes support for PV xenfb (included)

[Markus Armbruster <armbru@xxxxxxxxxx>]

Pat Campbell <plc@xxxxxxxxxx> writes:

> Markus Armbruster wrote:
[...]
>> I can't see how using grants for the page directory is an improvement
>> over your previous iteration, which used pd[].  Can you explain?
>>   
> I will try, I am not the greatest communicator lately. 
>
> Previous iteration did use pd[] extending it to 3 from 2, this had the
> drawback of limiting our frame buffer size to a max of 6 MB for a 64 bit
> guest, 12mb for a 32 bit guest.  6MB can handle a maximum of 1280x1024
> which in previous emails was deemed not enough. 
> HD 1080  1920x1200 requires   9MB   (pd[5], 64bit)
> WQXGA  2560x1600 requires 16MB   (pd[8], 64bit)
>
> I could of just extended pd[] some more to increase the available slots
> but every time we want to increase the total frame buffer size we have
> to compile both ends.  Probably could just keep reading/mapping until we
> hit a NULL in the backend to eliminate that issue but that ends adding
> any new items to the shared info page.

Every slot in pd[] gets us 2M (64 bit).  The shared page has space
left for about 500 slots.  We can easily increase pd[] to 256 slots,
good for 512M of framebuffer, or 128 Megapixels (>12000x10000), *and*
leave plenty of space in the shared page for future uses.

The backend should map precisely as much framebuffer as the frontend
tells it (struct xenfb_page member mem_length), using as much of pd[]
as necessary for that.

> By using grants:
>    I don't effect the current shared info
>    Don't have to deal with differences between a 64 bit long and a 32
> bit long. 
>    Single grant ref fits within the event structure
>
> Are those benefits worth the code complexity that grants added?   Not
> sure.  Other than grants, which I can remove and revert to pd[], are all
> the other issues that were raised addressed?

I haven't reviewed your patch in detail.  The change in fb mapping
stuck out, and I wanted it discussed before I do that.  Assuming we
can agree to extending pd[] instead, would you mind preparing a new
patch for us to review?

[...]

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