Messages in this thread | | | Date | Wed, 8 May 2013 10:35:08 -0400 | From | Konrad Rzeszutek Wilk <> | Subject | Re: WT memory type on x86_64? |
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On Wed, Apr 24, 2013 at 12:33:33PM -0700, Andy Lutomirski 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? > > If not, then here's a proposal: At startup, if there are no WC or WP > MTRRs and the CPU is new enough, then set a flag for an alternative > PAT. In alternative PAT mode, UC- is replaced with WT in the PAT and > mtrr_add fails when attempting to add a WC or WP entry. Add > set_memory_wt, etc. They fail in non-alternative-PAT mode. This gets > a bit unpleasant, since _PAGE_CACHE_UC_MINUS will have a different > meaning depending on the mode. > > A less conservative proposal would be to stop using UC- in the PAT > entirely. The memtype code could learn to emulate UC- when there's an > overlapping WC or WP MTRR. > > Any thoughts? Are there known errata here? Is there a good reason > why the kernel needs UC-? Will this be such a big can of worms that I > should just hope there's an available MTRR?
Stefan Bader (CC-ed here) was looking in making a "black-box" type code wherein for any types but WC it would lookup in the PAT index the right bit and provide that for the page_attr_set functions.
If I recall he had run in a bit of issue with of the hard-coded values set by the macros.
> > --Andy > > [1] I hope to be able to upstream all kernel code related to this > project. It will be neat -- I promise. It will depend on convincing > the people on the other end of the NDA that they should let me. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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