Re: WT memory type on x86_64?

From: Dave Airlie
Date: Fri Apr 26 2013 - 21:01:11 EST

On Sat, Apr 27, 2013 at 11:00 AM, Dave Airlie <airlied@xxxxxxxxx> wrote:
> On Sat, Apr 27, 2013 at 10:37 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Wed, Apr 24, 2013 at 12:33 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> For an upcoming (and, sadly, NDA'd [1]) project, I may need to use
>>> write-through memory. I'd like to gauge how unpleasant this will be.
>>> AFAICT, modern CPUs allow the WT type to be set using MTRR or a PAT
>>> entry. Sadly, MTRRs are in short supply, and the four fully-usable
>>> PAT slots are used for UC, UC-, WB, and WC. I can keep my fingers
>>> crossed and hope that there are enough free MTRRs, or I could try to
>>> free up a PAT entry.
>>> How nasty will the latter be? I just looked at two rather different
>>> modern Sandy Bridge machines, and BIOS doesn't appear to set up any
>>> MTRRs in the WC or WP states. As long as those MTRR types aren't
>>> used, I think the UC- PAT entry is useless -- it behaves identically
>>> to UC. Lots of DRM drivers, though, seen to add a WC MTRR to cover
>>> video memory. Is there any need for this on modern machines? That
>>> is, are there any drivers that actually need the mtrr_add call to
>>> succeed on a machine that has a working PAT?
>> FWIW, I've done a bit of a survey. Things that use UC or UC- include:
>> - ioremap_nocache: ISTM that any correct caller wants genuine UC memory.
>> - plain ioremap: Are there architectures where it's not
>> ioremap_nocache? (Tn any case, this is irrelevant.)
>> - pci_iomap: This is used all over the framebuffer code. It seems to
>> be equivalent to ioremap or ioremap_nocache, which are the same thing
>> on x86.
>> - AGP: The AGP code seems inconsistent. alloc_page gets a cacheable
>> page of RAM. alloc_pages gets uncached pages of RAM. In there's a WC
>> MTRR on RAM, then everything is screwed up anyway.
>> - ttm: This code is newish. I imagine that everything using ttm that
>> wants WC memory asks TTM for WC, which will work just fine. In any
>> case, the allocations are AFAICS backed by RAM, so there should be no
>> conflicts.
>> - radeon's gart: Ditto
>> - efi: presumably !WB means UC is fine. (Why would EFI need WC?)
>> - uvesafb: The MTRR code is terrifying. It looks nearly useless (it
>> has alignment issues) and it's unnecessary on a system with PAT. In
>> any case, this code certainly isn't expecting a WC MTRR with any kind
>> of mapping other than ioremap_wc.
>> mtrr_add users include:
>> - tdfxfb, vt8623fb, sgivwfb, s3fb, etc. should be converted to use ioremap_wc
>> - myri10ge tries to use an MTRR. This is, IMO, strange.
>> - Infiniband. I think it's okay if the MTRR doesn't work.
>> The only problematic (and not trivially fixable) thing I found is
>> pci_mmap_page_range, which uses UC- and is part of the ABI -- old X
>> drivers may care.
>> I wonder if X (using UMS) will slow down if WC MTRRs become illegal or
>> stop being added by old framebuffer drivers. (If so, they can be
>> randomly slow anyway -- lots of machines have no free MTRRs).
> Don't forget you can add mtrrs from userspace via /proc/mtrr. I'm not sure
> what sort ABI guarantees are on this.
> TTM allocations are not necessarily backed by RAM, they can also from
> device memory.
> Also i915 has mtrr code, but we avoid touching mtrrs if we are on a PAT cpu.

i915 also has this comment:
/* Set up a WC MTRR for non-PAT systems. This is more common than
* one would think, because the kernel disables PAT on first
* generation Core chips because WC PAT gets overridden by a UC
* MTRR if present. Even if a UC MTRR isn't present.

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