Re: Re: question about ioremap_cache and PAT

[Andy Lutomirski]

On Fri, Aug 23, 2013 at 9:59 AM, Andreas Werner <wernerandy@xxxxxx> wrote:
>
>
> Hi,
> thank you for your answer.
>
> So we are two persons for now who need WT :-)
>
> Im currently working on an ethernet driver for our own ETH core.
> The problem is that one requirement is to not use DMA to transmit or receive the data.
> This means the that the ethernet buffer are not located in the main memory. They are located in
> the FPGA.
>
> To transmit or receive a frame, i have to read or write to mmio to get the data.
>
> Intel has introduced the command "clflush" which can flush a cache line.
> I wanted to activate the caches for those mmio (eth buffer) to speed up the transmit or receive.
> After that the transfer over PCIe uses burst read/write.
>
> The problem was if i set the buffer to Write-Back and call clflush on those mmio-addresses, the system crashed without any output.
> I found this articel http://software.intel.com/en-us/forums/topic/393070.[http://software.intel.com/en-us/forums/topic/393070]
>
> After that i configured the transmit buffer to be Write-Combining (only write to that adresses) using ioremap_wc, and
> the receive buffer to be Write-Through (ioremap_cache + mtrr Write-Back + my Kernel Hack :-)) everything worked fine.
> The other configuration Register on the FPGA are just mapped with ioremap.

I'm curious: have you tried movntdqa on UC memory for this?
(Certainly WP or WT is easier.)

In any case, I hope to have patches to support WP and WT without using
PAT reasonably soon.

>
> On PCIe Tracer i can see the burst read/write on my device.
>
> Is it possible to get hits into the Kernel?
>
> My modification in arch/x86/mm/pat.c:
>
> --- pat.c.orig 2013-02-03 01:18:49.491879407 +0100
> +++ pat.c 2013-02-03 01:19:19.053509836 +0100
> @@ -149,10 +149,16 @@ static unsigned long pat_x_mtrr_type(u64
>    u8 mtrr_type;
>
>    mtrr_type = mtrr_type_lookup(start, end);
> -  if (mtrr_type != MTRR_TYPE_WRBACK)
> +
> +  if (mtrr_type == MTRR_TYPE_WRTHROUGH) {
> +   return _PAGE_CACHE_WB;
> +  }
> +  else if( mtrr_type == MTRR_TYPE_WRBACK )
> +   return _PAGE_CACHE_WB;
> +  else
>     return _PAGE_CACHE_UC_MINUS;
> -
> -  return _PAGE_CACHE_WB;
> +
>   }
>
>   return req_type;
>

That seems more or less reasonable to me.  If you want it included,
send it to x86@xxxxxxxxxx (cc lkml) and see what they say.

It would be prettier if you combined the conditions into a single
if/else, though.

>
> Best regards.
>
>
> Gesendet: Montag, 12. August 2013 um 19:53 Uhr
> Von: "Andy Lutomirski" <luto@xxxxxxxxxxxxxx>
> An: "Andreas Werner" <wernerandy@xxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Betreff: Re: question about ioremap_cache and PAT
> On 08/11/2013 09:50 AM, Andreas Werner wrote:
>> Hi i have a question about ioremap_cache and the resulting PAT attribute on X86 system. If I configure the mtrr to Write-Through for an adress range, and call ioremap_cache to map the mmio, the resulting PAT attribute is set to UC.
>> If I check the Intel document IA-32 SDM vol 3a, the resulting PAT attribute should be WB.
>>
>> I found the function pat_x_mtrr_type in arch/x86/mm/pat.c where the resulting attribute is returned. There will be always UC return expect if the MTRR is set to WB.
>>
>> Why is there only WB or UC returned? In the Intel document there are a lot of combinations "allowed".
>>
>> I need a Attribute of WT, so what i did is to modify the pat_x_mtrr_type function to return also WB if the MTRR is set to WT.
>>
>> Is this a solution to solve that or whats the reasion why the kernel doesn´t support this combination?
>
> The kernel doesn't support it because I'm apparently the only person who
> ever wanted it and I haven't implemented it yet.
>
> This stuff is handled in hardware, so modifying the kernel's idea of
> what hardware does won't do much. Also, the kernel using MTRRs is on
> its (very slow) way out. You could probably hack something up, but I
> can almost guarantee that hpa, etc won't accept the patches.
>
> That being said, I'm planning to support WT directly using PAT in the
> near future. This will work on most recent cpus (there are errata that
> will prevent use of the high PAT entries on some cpus).
>
> What do you need WT for? I want it for NVDIMMs, and all I need to get
> started now is a heatsink*, so I'll hopefully start implementing this
> stuff in the next week or so.
>
> --Andy
>
> * Damnit, Intel, it's not 2003 any more. You already figured out that
> heatsinks want screw holes. But why couldn't you make sure that all
> so-called "LGA 2011" sockets have the screw holes in the same place?
>
>
>>
>> Best regards
>>
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>> Best regards
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>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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