Date: Mon, 10 Apr 2017 12:42:35 +0200 From: "PaX Team" <pageexec@...email.hu> To: Andy Lutomirski <luto@...nel.org> CC: Andy Lutomirski <luto@...nel.org>, Mathias Krause <minipli@...glemail.com>, Thomas Gleixner <tglx@...utronix.de>, Kees Cook <keescook@...omium.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Mark Rutland <mark.rutland@....com>, Hoeun Ryu <hoeun.ryu@...il.com>, Emese Revfy <re.emese@...il.com>, Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() On 9 Apr 2017 at 17:10, Andy Lutomirski wrote: > On Sun, Apr 9, 2017 at 5:47 AM, PaX Team <pageexec@...email.hu> wrote: > > on x86 the cost of the pax_open/close_kernel primitives comes from the cr0 > > writes and nothing else, use_mm suffers not only from the cr3 writes but > > also locking/atomic ops and cr4 writes on its path and the inevitable TLB > > entry costs. and if cpu vendors cared enough, they could make toggling cr0.wp > > a fast path in the microcode and reduce its overhead by an order of magnitude. > > > > If the CR4 writes happen in for this use case, that's a bug. that depends on how you plan to handle perf/rdpmc users and how many alternative mm structs you plan to manage (one global, one per cpu, one per mm struct, etc). > > you'll be duplicating TLB entries in the alternative PCID for both code > > and data, where they will accumulate (=take room away from the normal PCID > > and expose unwanted memory for access) unless you also flush them when > > switching back (which then will cost even more cycles). also i'm not sure > > that processors implement all the 12 PCID bits so depending on how many PCIDs > > you plan to use, you could be causing even more unnecessary TLB replacements. > > > > Unless the CPU is rather dumber than I expect, the only duplicated > entries should be for the writable aliases of pages that are written. > The rest of the pages are global and should be shared for all PCIDs. well, 126.96.36.199 has language like this (188.8.131.52 implies similar): A logical processor may use a global TLB entry to translate a linear address, even if the TLB entry is associated with a PCID different from the current PCID. that to me says that global page entries are associated with a PCID and may (not) be used while in another PCID. in Intel-speak that's not 'dumb' but "tricks up our sleeve that we don't really want to tell you about in detail, except perhaps under a NDA".
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