Date: Sun, 13 Nov 2016 10:50:19 -0800 From: Adrian Chadd <adrian.chadd@gmail.com> To: Konstantin Belousov <kostikbel@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: <CAJ-Vmo=C-pqY4o-r2RXD-m0OxWeDaLpm_zUroeMpwZvLLuuZdg@mail.gmail.com> In-Reply-To: <CAJ-VmomG6e8WVyyuqAkC20fwZ5wX2hnwSE7T4r%2BTSDF%2BOzLCNQ@mail.gmail.com> References: <201610191109.u9JB9TTC002727@repo.freebsd.org> <CAJ-Vmom5rYe89m7bch4qoHHq3X2e67pPk_7G2aRGrjSPNp5mzg@mail.gmail.com> <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>
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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?
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.
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.
Thanks,
-adrian
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