Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support

[Guo Ren <guoren@kernel.org>]

Hi Anup and Atish,

On Thu, Jun 3, 2021 at 2:00 PM Anup Patel <Anup.Patel@wdc.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Palmer Dabbelt <palmer@dabbelt.com>
> > Sent: 03 June 2021 09:43
> > To: guoren@kernel.org
> > Cc: anup@brainfault.org; drew@beagleboard.org; Christoph Hellwig
> > <hch@lst.de>; Anup Patel <Anup.Patel@wdc.com>; wefu@redhat.com;
> > lazyparser@gmail.com; linux-riscv@lists.infradead.org; linux-
> > kernel@vger.kernel.org; linux-arch@vger.kernel.org; linux-
> > sunxi@lists.linux.dev; guoren@linux.alibaba.com; Paul Walmsley
> > <paul.walmsley@sifive.com>
> > Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support
> >
> > On Sat, 29 May 2021 17:30:18 PDT (-0700), Palmer Dabbelt wrote:
> > > On Fri, 21 May 2021 17:36:08 PDT (-0700), guoren@kernel.org wrote:
> > >> On Wed, May 19, 2021 at 3:15 PM Anup Patel <anup@brainfault.org>
> > wrote:
> > >>>
> > >>> On Wed, May 19, 2021 at 12:24 PM Drew Fustini
> > <drew@beagleboard.org> wrote:
> > >>> >
> > >>> > On Wed, May 19, 2021 at 08:06:17AM +0200, Christoph Hellwig
> > wrote:
> > >>> > > On Wed, May 19, 2021 at 02:05:00PM +0800, Guo Ren wrote:
> > >>> > > > Since the existing RISC-V ISA cannot solve this problem, it is
> > >>> > > > better to provide some configuration for the SOC vendor to
> > customize.
> > >>> > >
> > >>> > > We've been talking about this problem for close to five years.
> > >>> > > So no, if you don't manage to get the feature into the ISA it
> > >>> > > can't be supported.
> > >>> >
> > >>> > Isn't it a good goal for Linux to support the capabilities present
> > >>> > in the SoC that a currently being fab'd?
> > >>> >
> > >>> > I believe the CMO group only started last year [1] so the RV64GC
> > >>> > SoCs that are going into mass production this year would not have
> > >>> > had the opporuntiy of utilizing any RISC-V ISA extension for
> > >>> > handling cache management.
> > >>>
> > >>> The current Linux RISC-V policy is to only accept patches for frozen
> > >>> or ratified ISA specs.
> > >>> (Refer, Documentation/riscv/patch-acceptance.rst)
> > >>>
> > >>> This means even if emulate CMO instructions in OpenSBI, the Linux
> > >>> patches won't be taken by Palmer because CMO specification is still
> > >>> in draft stage.
> > >> Before CMO specification release, could we use a sbi_ecall to solve
> > >> the current problem? This is not against the specification, when CMO
> > >> is ready we could let users choose to use the new CMO in Linux.
> > >>
> > >> From a tech view, CMO trap emulation is the same as sbi_ecall.
> > >>
> > >>>
> > >>> Also, we all know how much time it takes for RISCV international to
> > >>> freeze some spec. Judging by that we are looking at another
> > >>> 3-4 years at minimum.
> > >
> > > Sorry for being slow here, this thread got buried.
> > >
> > > I've been trying to work with a handful of folks at the RISC-V
> > > foundation to try and get a subset of the various in-development
> > > specifications (some simple CMOs, something about non-caching in the
> > > page tables, and some way to prevent speculative accesse from
> > > generating coherence traffic that will break non-coherent systems).
> > > I'm not sure we can get this together quickly, but I'd prefer to at
> > > least try before we jump to taking vendor-specificed behavior here.
> > > It's obviously an up-hill battle to try and get specifications through
> > > the process and I'm certainly not going to promise it will work, but
> > > I'm hoping that the impending need to avoid forking the ISA will be
> > > sufficient to get people behind producing some specifications in a timely
> > fashion.
> > >
> > > I wasn't aware than this chip had non-coherent devices until I saw
> > > this thread, so we'd been mostly focused on the Beagle V chip.  That
> > > was in a sense an easier problem because the SiFive IP in it was never
> > > designed to have non-coherent devices so we'd have to make anything
> > > work via a series of slow workarounds, which would make emulating the
> > > eventually standardized behavior reasonable in terms of performance
> > > (ie, everything would be super slow so who really cares).
> > >
> > > I don't think relying on some sort of SBI call for the CMOs whould be
> > > such a performance hit that it would prevent these systems from being
> > > viable, but assuming you have reasonable performance on your
> > > non-cached accesses then that's probably not going to be viable to
> > > trap and emulate.  At that point it really just becomes silly to
> > > pretend that we're still making things work by emulating the
> > > eventually ratified behavior, as anyone who actually tries to use this
> > > thing to do IO would need out of tree patches.  I'm not sure exactly
> > > what the plan is for the page table bits in the specification right
> > > now, but if you can give me a pointer to some documentation then I'm
> > > happy to try and push for something compatible.
> > >
> > > If we can't make the process work at the foundation then I'd be
> > > strongly in favor of just biting the bullet and starting to take
> > > vendor-specific code that's been implemented in hardware and is
> > > necessarry to make things work acceptably.  That's obviously a
> > > sub-optimal solution as it'll lead to a bunch of ISA fragmentation,
> > > but at least we'll be able to keep the software stack together.
> > >
> > > Can you tell us when these will be in the hands of users?  That's
> > > pretty important here, as I don't want to be blocking real users from
> > > having their hardware work.  IIRC there were some plans to distribute
> > > early boards, but it looks like the foundation got involved and I
> > > guess I lost the thread at that point.
> > >
> > > Sorry this is all such a headache, but hopefully we can get things
> > > sorted out.
> >
> > I talked with some of the RISC-V foundation folks, we're not going to have an
> > ISA specification for the non-coherent stuff any time soon.  I took a look at
> > this code and I definately don't want to take it as is, but I'm not opposed to
> > taking something that makes the hardware work as long as it's a lot cleaner.
> > We've already got two of these non-coherent chips, I'm sure more will come,
> > and I'd rather have the extra headaches than make everyone fork the software
> > stack.
>
> Thanks for confirming. The CMO extension is still in early stages so it will
> certainly take more time for them. After CMO extension is finalized, it will
> take some more time to have actual RISC-V platforms with CMO implemented.
>
> >
> > After talking to Atish it looks like there's likely to be an SBI extension to
> > handle the CMOs, which should let us avoid the bulk of the vendor-specific
> > behavior in the kernel.  I know some people are worried about adding to the
> > SBI surface.  I'm worried about that too, but that's way better than sticking a
> > bunch of vendor-specific instructions into the kernel.  The SBI extension
> > should make for a straight-forward cache flush implementation in Linux, so
> > let's just plan on that getting through quickly (as has been done before).
>
> Yes, I agree. We can have just a single SBI call which is meant for DMA sync
> purpose only which means it will flush/invalidate pages from all cache
> levels irrespective of the cache hierarchy (i.e. flush/invalidate to RAM). The
> CMO extension might more generic cache operations which can target
> any cache level.
>
> I am already preparing a write-up for SBI DMA sync call in SBI spec. I will
> share it with you separately as well.
>
> >
> > Unfortunately we've yet to come up with a way to handle the non-cacheable
> > mappings without introducing a degree of vendor-specific behavior or
> > seriously impacting performance (mark them as not valid and deal with them
> > in the trap handler).  I'm not really sure it counts as supporting the hardware
> > if it's massively slow, so that really leaves us with vendor-specific mappings as
> > the only option to make these chips work.
>
> A RISC-V platform can have non-cacheable mappings is following possible
> ways:
> 1) Fixed physical address range as non-cacheable using PMAs
> 2) Custom page table attributes
> 3) Svpmbt extension being defined by RVI
>
> Atish and me both think it is possible to have RISC-V specific DMA ops
> implementation which can handle all above case. Atish is already working
> on DMA ops implementation for RISC-V.
Not only DMA ops, but also icache_sync & __vdso_icache_sync. Please
have a look at:
https://lore.kernel.org/linux-riscv/1622970249-50770-12-git-send-email-guoren@kernel.org/T/#u


>
> >
> > This implementation, which adds some Kconfig entries that control page table
> > bits, definately isn't suitable for upstream.  Allowing users to set arbitrary
> > page table bits will eventually conflict with the standard, and is just going to
> > be a mess.  It'll also lead to kernels that are only compatible with specific
> > designs, which we're trying very hard to avoid.  At a bare minimum we'll need
> > some way to detect systems with these page table bits before setting them,
> > and some description of what the bits actually do so we can reason about
> > them.
>
> Yes, vendor specific Kconfig options are strict NO NO. We can't give-up the
> goal of unified kernel image for all platforms.
>
> Regards,
> Anup



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/