Re: [PATCH] xen/video: Set EFI framebuffer to WC by default

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 06/11/2015 01:35 PM, Jan Beulich wrote:
>>>> On 11.06.15 at 14:09, <ross.lagerwall@citrix.com> wrote:
>> Set the EFI framebuffer to write-combining by default. This makes
>> booting somewhat faster, but more importantly avoids tripping the
>> watchdog. In particular, before on my test machine, each frame redraw
>> would take around 80ms, which can trip the 5s watchdog when constructing
>> dom0, since it outputs something like 60 lines without processing
>> pending softirqs.
>
> That would need fixing then. What are those 60 lines?

I think it's the lines from "Testing NMI watchdog on all CPUs: ok" to 
"Scrubbing Free RAM".

>
>> Both Linux and FreeBSD map the EFI framebuffer as write-combining by
>> default, so I assume (hope) that this is a safe change to make.
>
> No, an unaware Dom0 OS may not work correctly when the frame
> buffer is WC. It also might come as a surprise to the Dom0 OS that
> there is a WC range in one of the MTRRs where none would be
> expected. Plus - why for EFI only?

Because that's what other OSes did, but if it's not going to work... I 
can send a patch which we're currently using which sticks in a few 
process_pending_softirqs() in strategic places.

-- 
Ross Lagerwall