Date: Tue, 18 Dec 2018 07:27:38 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Alexander Lochmann <alexander.lochmann@tu-dortmund.de> Cc: freebsd-stable@freebsd.org, Horst Schirmeier <horst.schirmeier@tu-dortmund.de> Subject: Re: Address Collision using i386 4G/4G Memory Split Message-ID: <20181218052738.GZ60291@kib.kiev.ua> In-Reply-To: <38ad0d50-c776-9deb-d56b-db8db548cefc@tu-dortmund.de> References: <38ad0d50-c776-9deb-d56b-db8db548cefc@tu-dortmund.de>
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On Mon, Dec 17, 2018 at 02:51:48PM +0100, Alexander Lochmann wrote: > Hi folks! > > According to git commit e3089a (https://reviews.freebsd.org/D1463) > FreeBSD 12.0 i386 uses separate address spaces for kernel and user > space. So basically two memory areas, one in each space, can have the > same address. > Is this possible with FreeBSD 12.0? Is this likely to happen? The feature was added to HEAD during this summer, before stable/12 was branched. > > On my opinion, this is also very expensive in terms of performance. > Any copy{in,out} has to flush the TLB. > (http://fxr.watson.org/fxr/source/i386/i386/copyout_fast.s#L91) > Why are you still using this 4G/4G approach? Because it is needed for i386 to self-host, in modern world 1G KVA is too small, and because it provides Meltdown mitigation.
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