linux-kernel - Re: [PATCH 0/7] riscv: Memory Hot(Un)Plug support

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Date:   Wed, 17 May 2023 20:53:55 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     David Hildenbrand <david@...hat.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org
Cc:     Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Oscar Salvador <osalvador@...e.de>,
        virtualization@...ts.linux-foundation.org, linux@...osinc.com,
        Alexandre Ghiti <alexghiti@...osinc.com>
Subject: Re: [PATCH 0/7] riscv: Memory Hot(Un)Plug support

David Hildenbrand <david@...hat.com> writes:

> On 12.05.23 16:57, Björn Töpel wrote:
>> From: Björn Töpel <bjorn@...osinc.com>
>> 
>> Memory Hot(Un)Plug support for the RISC-V port
>> ==============================================

[...]

>
> Cool stuff! I'm fairly busy right now, so some high-level questions upfront:

No worries, and no rush! I'd say the v1 series was mainly for the RISC-V
folks, and I've got tons of (offline) comments from Alex -- and with
your comments below some more details to figure out.

> What is the memory section size (which implies the memory block size 
> and)? This implies the minimum DIMM granularity and the high-level 
> granularity in which virtio-mem adds memory.

It's 128M (27 bits) -- (like arm64 and x86-64?).

> What is the pageblock size, implying the minimum granularity that 
> virtio-mem can operate on?

Nothing special AFAIU; MAX_ORDER is 10, so PAGE_SIZE (4K) * 1024. Hmm, I
realize that I need to look into some more details of virtio-mem! :-)

> On x86-64 and arm64 we currently use the ACPI SRAT to expose the maximum 
> physical address where we can see memory getting hotplugged. [1] From 
> that, we can derive the "max_possible_pfn" and prepare the kernel 
> virtual memory layourt (especially, direct map).
>
> Is something similar required on RISC-V?

Yes! RISC-V is in the progress of getting proper ACPI support. Thanks
for pointing me in the these directions; Food for thought that I'll
digest for the next version.


Cheers,
Björn