linux-arm-kernel.lists.infradead.org archive mirror
 help / color / mirror / Atom feed
From: Jason Gunthorpe <jgg@nvidia.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Leon Romanovsky <leon@kernel.org>,
	linux-pci@vger.kernel.org, Bjorn Helgaas <helgaas@kernel.org>,
	catalin.marinas@arm.com, will@kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Clint Sbisa <csbisa@amazon.com>
Subject: Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs
Date: Thu, 10 Sep 2020 20:29:38 -0300	[thread overview]
Message-ID: <20200910232938.GJ904879@nvidia.com> (raw)
In-Reply-To: <44acc22377958a57c738f5139c5b5df2841c2544.camel@kernel.crashing.org>

On Fri, Sep 11, 2020 at 07:46:47AM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2020-09-10 at 14:10 -0300, Jason Gunthorpe wrote:
> > Can you explain what this actually does on ARM? 
> > 
> > Can it ever speculate loads across page boundaries, or speculate
> > loads
> > that never exist in the program? ie will we get random unpredicable
> > MemRds?
> 
> Probably, at least on powerpc you will as well, that's the only way to
> get write combine.

If I remove the PROT_READ in the user space mmap will it block it?

Read TLPs are not harmful but I suspect they would cause an
undesirable random performance anomaly.

> > Does it/could it "combine writes"?
> 
> I assume so for ARM, definitely for powerpc.

Various IBM PPC chips I know work, we do test that.

> > > That's why I looped you in - that's what worries me about
> > > "enabling"
> > > arch_can_pci_mmap_wc() on arm64. If we enable it and we have perf
> > > regressions that's not OK.
> > > 
> > > Or we *can* enable arch_can_pci_mmap_wc() but force the mellanox
> > > driver (or more broadly all drivers following this message push
> > > semantics) to use "something else" for WC detection.
> > 
> > arch_can_pci_mmap_wc() really only controls the sysfs resource file
> > and it seems very unclear who in userspace uses that these days.
> 
> dpdk under some circumstances afaik.

And something gross for DMA then? Not sure dpdk is useful without
DMA. Why not use CONFIG_VFIO_NOIOMMU for such a non-secure thing?
 
> > vfio is now the right way to do that stuff. I don't see an obvious
> > way to get WC memory in VFIO though...
> 
> Which would be a performance issue on a number of things I suppose...

Almost nothing uses pci_iomap_wc(), so I'd be surpried if userspace
DPDK was an important user when an in-kernel driver for the same HW
doesn't use it?

Jason

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2020-09-10 23:31 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200831151827.pumm2p54fyj7fz5s@amazon.com>
     [not found] ` <20200902113207.GA27676@e121166-lin.cambridge.arm.com>
     [not found]   ` <20200902142922.xc4x6m33unkzewuh@amazon.com>
2020-09-02 16:47     ` [PATCH] arm64: Enable PCI write-combine resources under sysfs Lorenzo Pieralisi
2020-09-02 17:21       ` Catalin Marinas
2020-09-02 17:54         ` Lorenzo Pieralisi
2020-09-02 23:03           ` Benjamin Herrenschmidt
2020-09-02 23:08         ` Benjamin Herrenschmidt
2020-09-02 23:08           ` Benjamin Herrenschmidt
2020-09-02 23:07       ` Benjamin Herrenschmidt
2020-09-03 11:08         ` Lorenzo Pieralisi
2020-09-03 14:36           ` Clint Sbisa
2020-09-03 22:26           ` Benjamin Herrenschmidt
2020-09-07 23:33           ` Benjamin Herrenschmidt
2020-09-10  9:46             ` Lorenzo Pieralisi
2020-09-10 10:54               ` Leon Romanovsky
2020-09-10 12:37               ` Jason Gunthorpe
2020-09-10 15:17                 ` Lorenzo Pieralisi
2020-09-10 17:10                   ` Jason Gunthorpe
2020-09-10 21:46                     ` Benjamin Herrenschmidt
2020-09-10 23:29                       ` Jason Gunthorpe [this message]
2020-09-11  0:39                         ` Benjamin Herrenschmidt
2020-09-11 14:21                           ` Jason Gunthorpe
2020-09-11 21:42                           ` Clint Sbisa
2020-09-14 14:17                             ` Jason Gunthorpe
2020-09-14 14:24                               ` Clint Sbisa
2020-09-14 14:38                                 ` Jason Gunthorpe
2020-09-14 21:42                                   ` Benjamin Herrenschmidt
2020-09-14 22:00                                     ` Benjamin Herrenschmidt
2020-09-14 22:32                                       ` Clint Sbisa
2020-09-14 22:57                                       ` Jason Gunthorpe
2020-09-14 23:25                                         ` Benjamin Herrenschmidt
2020-09-15 10:18                                           ` Lorenzo Pieralisi
2020-09-15 11:05                                             ` Jason Gunthorpe
2020-09-15 23:17                                               ` Benjamin Herrenschmidt
2020-09-15 23:40                                                 ` Jason Gunthorpe
2020-09-16  7:59                                                   ` Benjamin Herrenschmidt
2020-09-16 12:12                                                     ` Jason Gunthorpe
2020-09-16 14:09                                                       ` Lorenzo Pieralisi
2020-09-16 14:14                                                         ` Jason Gunthorpe
2020-09-16 23:59                                                       ` Benjamin Herrenschmidt
2020-09-17 10:28                                                         ` Lorenzo Pieralisi
2020-09-17 11:32                                                           ` Jason Gunthorpe
2020-09-17 14:01                                                             ` Lorenzo Pieralisi
2020-09-17 16:08                                                               ` Will Deacon
2020-09-16 12:48                                                     ` Leon Romanovsky
2020-09-16  8:33                                                   ` Will Deacon
2020-09-16  8:48                                                     ` Catalin Marinas
2020-09-16 14:15                                                       ` Lorenzo Pieralisi
2020-09-16 17:00                                                         ` Catalin Marinas
2020-09-16 21:29                                                           ` Benjamin Herrenschmidt
2020-09-16 12:08                                                     ` Jason Gunthorpe
2020-09-15 23:00                                             ` Benjamin Herrenschmidt
2020-09-15 23:12                                               ` Clint Sbisa
2020-09-14 21:41                               ` Benjamin Herrenschmidt
2020-08-21 15:51 Clint Sbisa
2020-08-27 14:41 ` Clint Sbisa
2020-08-31 15:22 ` Clint Sbisa

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200910232938.GJ904879@nvidia.com \
    --to=jgg@nvidia.com \
    --cc=benh@kernel.crashing.org \
    --cc=catalin.marinas@arm.com \
    --cc=csbisa@amazon.com \
    --cc=helgaas@kernel.org \
    --cc=leon@kernel.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-pci@vger.kernel.org \
    --cc=lorenzo.pieralisi@arm.com \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).