From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Palmer Dabbelt <palmer@dabbelt.com>, alex@ghiti.fr
Cc: aou@eecs.berkeley.edu, linux-mm@kvack.org, mpe@ellerman.id.au,
Anup Patel <Anup.Patel@wdc.com>,
linux-kernel@vger.kernel.org, Atish Patra <Atish.Patra@wdc.com>,
paulus@samba.org, zong.li@sifive.com,
Paul Walmsley <paul.walmsley@sifive.com>,
linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone
Date: Wed, 22 Jul 2020 09:12:58 +1000 [thread overview]
Message-ID: <6fbea8347bdb8434d91cf3ec2b95b134bd66cfe3.camel@kernel.crashing.org> (raw)
In-Reply-To: <mhng-08bff01a-ca15-4bbc-8454-2ca3e823fef8@palmerdabbelt-glaptop1>
On Tue, 2020-07-21 at 12:05 -0700, Palmer Dabbelt wrote:
>
> * We waste vmalloc space on 32-bit systems, where there isn't a lot of it.
> * On 64-bit systems the VA space around the kernel is precious because it's the
> only place we can place text (modules, BPF, whatever).
Why ? Branch distance limits ? You can't use trampolines ?
> If we start putting
> the kernel in the vmalloc space then we either have to pre-allocate a bunch
> of space around it (essentially making it a fixed mapping anyway) or it
> becomes likely that we won't be able to find space for modules as they're
> loaded into running systems.
I dislike the kernel being in the vmalloc space (see my other email)
but I don't understand the specific issue with modules.
> * Relying on a relocatable kernel for sv48 support introduces a fairly large
> performance hit.
Out of curiosity why would relocatable kernels introduce a significant
hit ? Where about do you see the overhead coming from ?
> Roughly, my proposal would be to:
>
> * Leave the 32-bit memory map alone. On 32-bit systems we can load modules
> anywhere and we only have one VA width, so we're not really solving any
> problems with these changes.
> * Staticly allocate a 2GiB portion of the VA space for all our text, as its own
> region. We'd link/relocate the kernel here instead of around PAGE_OFFSET,
> which would decouple the kernel from the physical memory layout of the system.
> This would have the side effect of sorting out a bunch of bootloader headaches
> that we currently have.
> * Sort out how to maintain a linear map as the canonical hole moves around
> between the VA widths without adding a bunch of overhead to the virt2phys and
> friends. This is probably going to be the trickiest part, but I think if we
> just change the page table code to essentially lie about VAs when an sv39
> system runs an sv48+sv39 kernel we could make it work -- there'd be some
> logical complexity involved, but it would remain fast.
>
> This doesn't solve the problem of virtually relocatable kernels, but it does
> let us decouple that from the sv48 stuff. It also lets us stop relying on a
> fixed physical address the kernel is loaded into, which is another thing I
> don't like.
>
> I know this may be a more complicated approach, but there aren't any sv48
> systems around right now so I just don't see the rush to support them,
> particularly when there's a cost to what already exists (for those who haven't
> been watching, so far all the sv48 patch sets have imposed a significant
> performance penalty on all systems).
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next prev parent reply other threads:[~2020-07-21 23:13 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-07 7:59 [PATCH v5 0/4] vmalloc kernel mapping and relocatable kernel Alexandre Ghiti
2020-06-07 7:59 ` [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone Alexandre Ghiti
2020-06-11 21:34 ` Atish Patra
2020-06-12 12:30 ` Alex Ghiti
2020-07-09 5:05 ` Palmer Dabbelt
2020-07-09 8:15 ` Zong Li
2020-07-09 11:11 ` Alex Ghiti
2020-07-21 18:36 ` Alex Ghiti
2020-07-21 19:05 ` Palmer Dabbelt
2020-07-21 23:12 ` Benjamin Herrenschmidt [this message]
2020-07-21 23:48 ` Palmer Dabbelt
2020-07-22 2:21 ` Benjamin Herrenschmidt
2020-07-22 4:50 ` Michael Ellerman
2020-07-22 5:46 ` Palmer Dabbelt
2020-07-22 9:43 ` Arnd Bergmann
2020-07-22 19:52 ` Palmer Dabbelt
2020-07-22 20:22 ` Arnd Bergmann
2020-07-22 21:05 ` Atish Patra
2020-07-24 7:20 ` Arnd Bergmann
2020-07-23 5:32 ` Alex Ghiti
2020-07-21 23:11 ` Benjamin Herrenschmidt
2020-07-21 23:36 ` Palmer Dabbelt
2020-07-23 5:36 ` Alex Ghiti
2020-07-23 5:21 ` Alex Ghiti
2020-07-23 22:33 ` Benjamin Herrenschmidt
2020-07-24 8:14 ` Arnd Bergmann
2020-06-07 7:59 ` [PATCH v5 2/4] riscv: Introduce CONFIG_RELOCATABLE Alexandre Ghiti
2020-06-10 14:10 ` Jerome Forissier
2020-06-11 19:43 ` Alex Ghiti
2020-06-07 7:59 ` [PATCH v5 3/4] powerpc: Move script to check relocations at compile time in scripts/ Alexandre Ghiti
2020-06-07 7:59 ` [PATCH v5 4/4] riscv: Check relocations at compile time Alexandre Ghiti
2020-07-08 4:21 ` [PATCH v5 0/4] vmalloc kernel mapping and relocatable kernel Alex Ghiti
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