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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.06.15 at 14:09, <ross.lagerwall@xxxxxxxxxx> 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?

> 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?

> --- a/xen/drivers/video/vesa.c
> +++ b/xen/drivers/video/vesa.c
> @@ -142,7 +142,13 @@ void __init vesa_init(void)
>  #include <asm/mtrr.h>
>  
>  static unsigned int vesa_mtrr;
> -integer_param("vesa-mtrr", vesa_mtrr);
> +static __initdata bool_t vesa_mtrr_set;
> +static void __init parse_vesa_mtrr(const char * s)
> +{
> +    vesa_mtrr = simple_strtoull(s, NULL, 0);
> +    vesa_mtrr_set = 1;
> +}
> +custom_param("vesa-mtrr", parse_vesa_mtrr);

I don't think any of this would be really needed if you simply
initialized vesa_mtrr to e.g. ~0.

Jan


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