linux-kernel - Re: [RFC PATCH 0/3] PCI/P2PMEM: introduce pci_p2pdma_align

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Message-ID: <4bbb0934-384a-533f-b964-6b3695719b25@redhat.com>
Date:   Wed, 2 Jun 2021 14:08:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Max Gurtovoy <mgurtovoy@...dia.com>,
        linux-nvme@...ts.infradead.org, dan.j.williams@...el.com,
        logang@...tatee.com, linux-mm@...ck.org, hch@....de
Cc:     sagi@...mberg.me, oren@...dia.com, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 0/3] PCI/P2PMEM: introduce pci_p2pdma_align_size API

On 02.06.21 13:10, Max Gurtovoy wrote:
> Hi all,
> In hotplugged memory (from check_pfn_span function):
> "
> Disallow all operations smaller than a sub-section and only
> allow operations smaller than a section for
> SPARSEMEM_VMEMMAP. Note that check_hotplug_memory_range()
> enforces a larger memory_block_size_bytes() granularity for
> memory that will be marked online, so this check should only
> fire for direct arch_{add,remove}_memory() users outside of
> add_memory_resource()
> "
> 
> This restriction will disqualify, for example, large NVMe CMBs that might have
> non power of 2 number of pages (e.g. 32767 pages of 4KB). For these
> devices, the CMB size will be rounded down from 0x7fff000 to 0x7e00000
> but it's better than having un-mapped CMB.

Just some high-level questions:

A CMB is just a PCI BAR used for communicating with the device, to be 
mapped into physical address space, right? I assume the relevant hotplug 
code is:

drivers/pci/p2pdma.c:   addr = devm_memremap_pages(&pdev->dev, pgmap);

correct?


Having a BAR span such weird sizes will most probably never be fully 
supported. But if sub-sections work for you, great.



-- 
Thanks,

David / dhildenb