linux-kernel - RE: CMA enhancement - non-default areas in x86

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Date:   Wed, 13 May 2020 09:43:45 +0000
From:   "Ravich, Leonid" <Leonid.Ravich@...l.com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "Idgar, Or" <Or.Idgar@...l.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: CMA enhancement - non-default areas in x86

> A: http://en.wikipedia.org/wiki/Top_post
> Q: Were do I find info about this thing called top-posting?
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
> 
> A: No.
> Q: Should I include quotations after my reply?
> 
> http://daringfireball.net/2007/07/on_top

Sorry , bad habit .
 
> On Wed, May 13, 2020 at 08:29:16AM +0000, Ravich, Leonid wrote:
> > PCIe NTB
> > Documentation/driver-api/ntb.rst
> 
> > 1) Basically PCI bridge between to root complex / PCI switches
> > 2) using out of OS memory is one solution but then this memory is
> > Limited for usage by other stack, ex: get_user_pages on this memory
> > will fail, Therefore attempting to use it for block layer with (o_direct) will
> fail.
> >
> > Acutely any generic stack which attempts to "pin" this memory will fail.
> 
> So why isn't the BIOS/UEFI properly reserving this from the general operating
> system's pages so that the driver knows to use them instead?
> 
> Is UEFI wrong here about these being valid memory ranges for general use?
> If so, why not fix that?  If not, how in the world is the OS supposed to know
> these memory ranges are _not_ for general use?  I feel like there is
> something missing here...
>
Maybe I am miss understanding something here , but if BIOS/UEFI will reserve this pages 
They will be "out of kernel" which will work for propriety driver but this memory will not 
be useable for generic driver which will attempt to pin this memory with get_user_pages() .
so we can go and try to fix that  (not sure this is the right way) .

another option here is to use some kernel infrastructure  which  from one side reserve the memory from general use
on the other hand kernel will be aware of this pages so get_user_pages()  will work on this memory .

from what we saw CMA infrastructure can support  such requirements.
Please let me know if you think I missing here something .

Thanks , and sorry for format mess .
> thanks,
> 
> greg k-h