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From:   "Ravich, Leonid" <Leonid.Ravich@dell.com>
To:     "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
CC:     "Idgar, Or" <Or.Idgar@Dell.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: RE: CMA enhancement - non-default areas in x86
Thread-Topic: CMA enhancement - non-default areas in x86
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Date:   Wed, 13 May 2020 09:43:45 +0000
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> 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?
>=20
> A: No.
> Q: Should I include quotations after my reply?
>=20
> http://daringfireball.net/2007/07/on_top

Sorry , bad habit .
=20
> On Wed, May 13, 2020 at 08:29:16AM +0000, Ravich, Leonid wrote:
> > PCIe NTB
> > Documentation/driver-api/ntb.rst
>=20
> > 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_direc=
t) will
> fail.
> >
> > Acutely any generic stack which attempts to "pin" this memory will fail=
.
>=20
> So why isn't the BIOS/UEFI properly reserving this from the general opera=
ting
> system's pages so that the driver knows to use them instead?
>=20
> 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 reserv=
e this pages=20
They will be "out of kernel" which will work for propriety driver but this =
memory will not=20
be useable for generic driver which will attempt to pin this memory with ge=
t_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 s=
ide reserve the memory from general use
on the other hand kernel will be aware of this pages so get_user_pages()  w=
ill 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,
>=20
> greg k-h