Date: Sat, 24 Jan 2015 12:21:05 -0800 From: Nathan Whitehorn <nwhitehorn@freebsd.org> To: Ian Lepore <ian@freebsd.org>, Alan Cox <alc@rice.edu> Cc: Konstantin Belousov <kostikbel@gmail.com>, svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r277643 - in head/sys: arm/arm dev/mem i386/i386 mips/mips sparc64/sparc64 Message-ID: <54C3FEB1.5020808@freebsd.org> In-Reply-To: <1422125623.1038.68.camel@freebsd.org> References: <201501241251.t0OCpGa8053192@svn.freebsd.org> <1422111397.1038.53.camel@freebsd.org> <20150124154240.GV42409@kib.kiev.ua> <54C3E563.4070903@rice.edu> <1422125623.1038.68.camel@freebsd.org>
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On 01/24/15 10:53, Ian Lepore wrote: > On Sat, 2015-01-24 at 12:33 -0600, Alan Cox wrote: >> On 01/24/2015 09:42, Konstantin Belousov wrote: >>> On Sat, Jan 24, 2015 at 07:56:37AM -0700, Ian Lepore wrote: >>>> On Sat, 2015-01-24 at 12:51 +0000, Konstantin Belousov wrote: >>>>> Author: kib >>>>> Date: Sat Jan 24 12:51:15 2015 >>>>> New Revision: 277643 >>>>> URL: https://svnweb.freebsd.org/changeset/base/277643 >>>>> >>>>> Log: >>>>> Remove Giant from /dev/mem and /dev/kmem. It is definitely not needed >>>>> for i386, and from the code inspection, nothing in the >>>>> arm/mips/sparc64 implementations depends on it. >>>>> >>>> I'm not sure I agree with that. On arm the memrw() implementation uses >>>> a single statically-allocated page of kva space into which it maps each >>>> physical page in turn in the main loop. What prevents preemption or >>>> multicore access to /dev/mem from trying to use that single page for >>>> multiple operations at once? >>> I see, thank you for noting this. >>> >>> But, I do not think that Giant is a solution for the problem. uiomove() >>> call accesses userspace, which may fault and cause sleep. If the >>> thread sleeps, the Giant is automatically dropped, so there is no real >>> protection. >>> >>> I think dump exclusive sx around whole memrw() should be enough. >>> >>> I can revert the commit for now, or I can leave it as is while >>> writing the patch with sx and waiting for somebody review. What >>> would you prefer ? >>> >>> P.S. mips uses uiomove_fromphys(), avoiding transient mapping, >>> and sparc allocates KVA when needed. >>> >>> >> While we're here, it's worth noting that the arm version of /dev/mem is >> not functionally equivalent to that of amd64 or i386. Arm disallows >> access to non-DRAM addresses through /dev/mem. > That's true for the read/write interface, but not for mmap(). In fact, > we have users insisting that mmap() on /dev/mem should provide userland > access to memory-mapped devices, and we have ARM architecture rules that > say you can't map the same physical address multiple times with > different attributes, such as being Device memory in the kernel and > Strongly Ordered when mapped into userland. But if the memory isn't > mapped S-O for userland, they have no hope of usefully accessing the > devices (because they don't have access to cache and buffer > maintenance). > > Even "normal" memory has a variety of attributes that make the temporary > mappings done in memrw() a bit iffy, although I'm not sure we're doing > anything right now that could lead to trouble. Trouble lurks though if > we ever start using some of the more subtle features of the arm memory > architecture, such as turning off the sharable attribute on pages that > are inherently per-cpu to avoid the overhead of hardware cache coherence > when we know only one core can access the pages. sparc64 also does not allow mmap() of device memory through /dev/mem for this reason. This leads to a whole bunch of #ifdef in X drivers. -Nathan
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