Re: [RFC PATCH 3/11] x86, mm, pat: Change reserve_memtype() to handle WT type

From: H. Peter Anvin
Date: Tue Jul 15 2014 - 19:47:21 EST

On 07/15/2014 04:36 PM, Andy Lutomirski wrote:
> On Tue, Jul 15, 2014 at 4:10 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
>> On Tue, 2014-07-15 at 12:56 -0700, Andy Lutomirski wrote:
>>> On Tue, Jul 15, 2014 at 12:34 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
>>>> This patch changes reserve_memtype() to handle the new WT type.
>>>> When (!pat_enabled && new_type), it continues to set either WB
>>>> or UC- to *new_type. When pat_enabled, it can reserve a given
>>>> non-RAM range for WT. At this point, it 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.
>>> FWIW, last time I looked at this, it seemed like all the fancy
>>> reserve_ram_pages stuff was unnecessary: shouldn't the RAM type be
>>> easy to track in the direct map page tables?
>> Are you referring the direct map page tables as the kernel page
>> directory tables (pgd/pud/..)?
>> I think it needs to be able to keep track of the memory type per a
>> physical memory range, not per a translation, in order to prevent
>> aliasing of the memory type.
> Actual RAM (the lowmem kind, which is all of it on x86_64) is mapped
> linearly somewhere in kernel address space. The pagetables for that
> mapping could be used as the canonical source of the memory type for
> the ram range in question.
> This only works for lowmem, so maybe it's not a good idea to rely on it.

We could do that, but would it be better?


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