Date: Sun, 13 Nov 2016 21:03:44 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Adrian Chadd <adrian.chadd@gmail.com> Cc: Juli Mallett <jmallett@freebsd.org>, Warner Losh <wlosh@bsdimp.com>, Warner Losh <imp@bsdimp.com>, "freebsd-mips@freebsd.org" <freebsd-mips@freebsd.org> Subject: Re: svn commit: r307626 - head/sys/ufs/ffs Message-ID: <20161113190344.GM54029@kib.kiev.ua> In-Reply-To: <CAJ-Vmo=C-pqY4o-r2RXD-m0OxWeDaLpm_zUroeMpwZvLLuuZdg@mail.gmail.com> References: <20161113065851.GD54029@kib.kiev.ua> <CANCZdfreg9wD-2pdGtzgeO86BptSE4M7LnLFTC09K3RHyHR8cQ@mail.gmail.com> <20161113071911.GF54029@kib.kiev.ua> <CANCZdfpC6smeNSPKzpbX8aAnF8CZ%2BSEFQmQ74jqvWUVXrttM%2BQ@mail.gmail.com> <20161113075557.GH54029@kib.kiev.ua> <71C512CD-0FB6-40D8-B46C-30467A245693@bsdimp.com> <CACVs6=_zmjJhMzmyFGJGHK1RAguQ_fZUcd94ZEmVEnTXBiOSdQ@mail.gmail.com> <20161113161548.GK54029@kib.kiev.ua> <CAJ-VmomG6e8WVyyuqAkC20fwZ5wX2hnwSE7T4r%2BTSDF%2BOzLCNQ@mail.gmail.com> <CAJ-Vmo=C-pqY4o-r2RXD-m0OxWeDaLpm_zUroeMpwZvLLuuZdg@mail.gmail.com>
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On Sun, Nov 13, 2016 at 10:50:19AM -0800, Adrian Chadd wrote: > Ok, so after talking with others, my questions are: > > * I thought our VM was supposed to not be doing double mapping like > this. warner's comment on irc was: > > === > 13:39 < bsdimp> adrian the VM isn't supposed to do it at all. > 13:39 < bsdimp> adrian that is, double map on purpose. > 13:40 < bsdimp> though there's some exceptions to the rule > 13:40 < bsdimp> since kernel mappings go away in userland, and > userland doesn't execute while you're in kernel mode, you can do the > flushing > game in busdma to prevent most issues. > 13:41 < bsdimp> which is what we do. Generally, though, our VM doesn't > do it in-kernel. > === > > * is this still the case? or are there places in the VM where we are doing this? > * can we introduce a machdep/pmap capability check to see if aliasing > is allowed and if so, turn this feature on? This never was the case, and never will. VM establishes mappings as requested by the userspace and kernel, and n-times mapping for the same page is always legitimate. It is the pmap duty to handle that. If you cared to read my previous mail, I already explained that besides the userspace asking for n-mapping, there is at least buffer cache which also maps the same page into KVA, from the times when unified buffer cache/page cache was introduced. Same is true for n-mapping of shared anon pages. > > Adding a pmap capability and turning it on for say, i386/amd64/arm64 > would allow for this new feature as well as the previous behaviour on > older platforms. > > I don't think I have the time to fix mips pmap to support this new > feature, so if you want to turn it on for all features, we should > really fix/test pmap on said platforms first. This is not a new feature, this is the old bug on that platform. > > Final comment: I'd really like to see a sort of "tested on" for things > like this, because it's not clear which platforms/architectures it was > tested on.
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