Messages in this thread | | | Subject | Re: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle WT | From | Toshi Kani <> | Date | Wed, 10 Sep 2014 13:30:28 -0600 |
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On Wed, 2014-09-10 at 11:26 -0700, Andy Lutomirski wrote: > On Wed, Sep 10, 2014 at 9:51 AM, Toshi Kani <toshi.kani@hp.com> wrote: > > This patch changes reserve_memtype() to handle the WT cache mode. > > When PAT is not enabled, it continues to set UC- to *new_type for > > any non-WB request. > > > > When a target range is RAM, reserve_ram_pages_type() fails for WT > > for now. This function may not reserve a RAM range for WT since > > reserve_ram_pages_type() uses the page flags limited to three memory > > types, WB, WC and UC. > > Should it fail if WT is unavailable due to errata? More generally, > how are all of the do_something_wc / do_something_wt / > do_something_nocache helpers supposed to handle unsupported types?
When WT is unavailable due to the PAT errata, it does not fail but gets redirected to UC-. Similarly, when PAT is disabled, WT gets redirected to UC- as well.
The failure case above is a run-time error when WT is enabled and is targeted to RAM. In this case, reserve_memtype() fails and sets UC- to *new_type due to the limitation in page tables. set_memory_xzy() interfaces do not retry with new_type, but return an error. I think this makes sense since the caller should receive this error as this case is a bug in the code (while running it on an old system is not a bug).
Thanks, -Toshi
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