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Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.1 #2 (Red Hat Linux)) id 1u4Hae-00000009f8d-1u5R; Mon, 14 Apr 2025 11:03:00 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id E6767300619; Mon, 14 Apr 2025 13:02:59 +0200 (CEST) Date: Mon, 14 Apr 2025 13:02:59 +0200 From: Peter Zijlstra To: Ingo Molnar Cc: Ankur Arora , linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, luto@kernel.org, paulmck@kernel.org, rostedt@goodmis.org, tglx@linutronix.de, willy@infradead.org, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH v3 1/4] x86/clear_page: extend clear_page*() for multi-page clearing Message-ID: <20250414110259.GF5600@noisy.programming.kicks-ass.net> References: <20250414034607.762653-1-ankur.a.arora@oracle.com> <20250414034607.762653-2-ankur.a.arora@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 7EDCE100011 X-Stat-Signature: gmcxtxae6zebyrrz4616xodgg985341c X-HE-Tag: 1744628590-305896 X-HE-Meta: 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 nkm0OQIG V1nVr7lcuA3j9kygqlj7O7hdp2JCHkOnqUYbi5tA/pp/QRG/MZhFlcwM0xJOh2GRgQb7ng2xoPNfDFCF5z7pwqu3JwxYkmiMxN4yxtK9DE319LMR+eIDfPznl4oGdHQwmWcMRHj00zc+fEP0G/40rYiLbtzYglLlTxuuRIJiEQjq1Ti/tY46r00kvWOqke/M6EXj0cxGuBogTss7hXq9a76CcFDgUOw1HTmekbCtKmEbtC8QPxLuOona+Dw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Apr 14, 2025 at 08:32:29AM +0200, Ingo Molnar wrote: > > static inline void clear_page(void *page) > > { > > + unsigned int length = PAGE_SIZE; > > /* > > - * Clean up KMSAN metadata for the page being cleared. The assembly call > > + * Clean up KMSAN metadata for the pages being cleared. The assembly call > > * below clobbers @page, so we perform unpoisoning before it. > > > */ > > - kmsan_unpoison_memory(page, PAGE_SIZE); > > - alternative_call_2(clear_page_orig, > > - clear_page_rep, X86_FEATURE_REP_GOOD, > > - clear_page_erms, X86_FEATURE_ERMS, > > + kmsan_unpoison_memory(page, length); > > + > > + alternative_call_2(clear_pages_orig, > > + clear_pages_rep, X86_FEATURE_REP_GOOD, > > + clear_pages_erms, X86_FEATURE_ERMS, > > "=D" (page), > > - "D" (page), > > + ASM_INPUT("D" (page), "S" (length)), > > "cc", "memory", "rax", "rcx"); > > } > > > > diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S > > index a508e4a8c66a..bce516263b69 100644 > > --- a/arch/x86/lib/clear_page_64.S > > +++ b/arch/x86/lib/clear_page_64.S > > @@ -13,20 +13,35 @@ > > */ > > > > /* > > - * Zero a page. > > - * %rdi - page > > + * Zero kernel page aligned region. > > + * > > + * Input: > > + * %rdi - destination > > + * %esi - length > > + * > > + * Clobbers: %rax, %rcx > > */ > > -SYM_TYPED_FUNC_START(clear_page_rep) > > - movl $4096/8,%ecx > > +SYM_TYPED_FUNC_START(clear_pages_rep) > > + movl %esi, %ecx > > xorl %eax,%eax > > + shrl $3,%ecx > > rep stosq > > RET > > -SYM_FUNC_END(clear_page_rep) > > -EXPORT_SYMBOL_GPL(clear_page_rep) > > +SYM_FUNC_END(clear_pages_rep) > > +EXPORT_SYMBOL_GPL(clear_pages_rep) > > > > -SYM_TYPED_FUNC_START(clear_page_orig) > > +/* > > + * Original page zeroing loop. > > + * Input: > > + * %rdi - destination > > + * %esi - length > > + * > > + * Clobbers: %rax, %rcx, %rflags > > + */ > > +SYM_TYPED_FUNC_START(clear_pages_orig) > > + movl %esi, %ecx > > xorl %eax,%eax > > - movl $4096/64,%ecx > > + shrl $6,%ecx > > So if the natural input parameter is RCX, why is this function using > RSI as the input 'length' parameter? Causes unnecessary register > shuffling. This symbol is written as a C function with C calling convention, even though it is only meant to be called from that clear_page() alternative. If we want to go change all this, then we should go do the same we do for __clear_user() and write it thusly: asm volatile(ALTERNATIVE("rep stosb", "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS) : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT : "a" (0)) And forget about all those clear_page_*() thingies.