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Message-ID: <7c3720ff-b763-44b0-9b57-a2fbff3c7f56@csgroup.eu>
Date: Fri, 6 Sep 2024 11:37:34 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Mike Rapoport <rppt@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Andreas Larsson <andreas@...sler.com>, Andy Lutomirski <luto@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
 Brian Cain <bcain@...cinc.com>, Catalin Marinas <catalin.marinas@....com>,
 Christoph Hellwig <hch@...radead.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Dinh Nguyen
 <dinguyen@...nel.org>, Geert Uytterhoeven <geert@...ux-m68k.org>,
 Guo Ren <guoren@...nel.org>, Helge Deller <deller@....de>,
 Huacai Chen <chenhuacai@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Johannes Berg <johannes@...solutions.net>,
 John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
 Kent Overstreet <kent.overstreet@...ux.dev>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Luis Chamberlain <mcgrof@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Masami Hiramatsu <mhiramat@...nel.org>, Matt Turner <mattst88@...il.com>,
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 Palmer Dabbelt <palmer@...belt.com>, Peter Zijlstra <peterz@...radead.org>,
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 Steven Rostedt <rostedt@...dmis.org>,
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 Thomas Gleixner <tglx@...utronix.de>, Uladzislau Rezki <urezki@...il.com>,
 Vineet Gupta <vgupta@...nel.org>, Will Deacon <will@...nel.org>,
 bpf@...r.kernel.org, linux-alpha@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
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 linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev,
 sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 7/8] execmem: add support for cache of large ROX pages



Le 26/08/2024 à 08:55, Mike Rapoport a écrit :
> From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> 
> Using large pages to map text areas reduces iTLB pressure and improves
> performance.
> 
> Extend execmem_alloc() with an ability to use PMD_SIZE'ed pages with ROX
> permissions as a cache for smaller allocations.

Why only PMD_SIZE ?

On power 8xx, PMD_SIZE is 4M and the 8xx doesn't have such a page size. 
When you call vmalloc() with VM_ALLOW_HUGE_VMAP you get 16k pages or 
512k pages depending on the size you ask for, see function 
arch_vmap_pte_supported_shift()

> 
> To populate the cache, a writable large page is allocated from vmalloc with
> VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> ROX.
> 
> Portions of that large page are handed out to execmem_alloc() callers
> without any changes to the permissions.
> 
> When the memory is freed with execmem_free() it is invalidated again so
> that it won't contain stale instructions.
> 
> The cache is enabled when an architecture sets EXECMEM_ROX_CACHE flag in
> definition of an execmem_range.
> 

Christophe

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