Re: [RFC Patch] use MTRR for write combining if PAT is not available

[Thomas S. <tho...@we...>]

Jan Beulich wrote:
> >>> Thomas Schlichter <tho...@we...> 12.10.09 20:32 >>>
> >+	if (!pat_enabled) {
> >+		void __iomem *ret = ioremap_nocache(phys_addr, size);
> >+		if (ret)
> >+			mtrr_add(phys_addr, size, MTRR_TYPE_WRCOMB, false);
> 
> This won't work if phys_addr or size aren't page aligned, or if size isn't
> a power of two.

No, at least the comments in mtrr_add and mtrr_check state that it is just 
required that phys_addr and size are multiple of PAGE_SIZE. And I'm not sure 
if it is always safe to round these up/down to the next PAGE boundary. If it 
is not, maybe it is better to fail...

> >+	if (!pat_enabled) {
> >+		ret = set_memory_uc(addr, numpages);
> >+		if (!ret)
> >+			mtrr_add(__pa(addr), numpages * PAGE_SIZE,
> >+				 MTRR_TYPE_WRCOMB, false);
> 
> Similarly, this requires numpages to be a power of two.

Same as above.

> I think user mode code handles this by splitting the request and trying to
> establish as many entries as possible (and those as big as possible).

Seems not required here...

> Also, in all cases you pass 'false' for the 'increment' parameter, in order
> to avoid having to tear down the established entries. While this may be
> reasonable for kernel initiated mappings, I don't think that's acceptable
> for such originating from user mode.

Well, therefore I would have to remember which mmap successfully set up which 
MTRR entries. And it must be ensured that for each mmap there always is a 
munmap. If this is indeed always ensured, and if it's worth to maintain the 
required data, I'd be happy to add a corresponding mtrr_del there.

Kind regards,
  Thomas