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

["H. Peter Anvin" <hpa@zytor.com>]

It already happened...

On July 15, 2014 5:28:40 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Tue, Jul 15, 2014 at 4:53 PM, Toshi Kani <toshi.kani@hp.com> wrote:
>> On Tue, 2014-07-15 at 16:36 -0700, Andy Lutomirski wrote:
>>> On Tue, Jul 15, 2014 at 4:10 PM, Toshi Kani <toshi.kani@hp.com>
>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@hp.com>
>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.
>>
>> Right.
>>
>> I think using struct page table for the RAM ranges is a good way for
>> saving memory, but I wonder how often the RAM ranges are mapped other
>> than WB...  If not often, reserve_memtype() could simply call
>> rbt_memtype_check_insert() for all ranges, including RAM.
>>
>> In this patch, I left using reserve_ram_pages_type() since I do not
>see
>> much reason to use WT for RAM, either.
>
>I hereby predict that someone, some day, will build a system with
>nonvolatile "RAM", and someone will want this feature.  Just saying :)
>
>More realistically, someone might want to write a silly driver that
>lets programs mmap some WT memory for testing.
>
>--Andy

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