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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-458705bccc5sm17738625e9.22.2025.07.24.05.00.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Jul 2025 05:00:38 -0700 (PDT) Date: Thu, 24 Jul 2025 13:00:37 +0100 From: David Laight To: Oleg Nesterov Cc: Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "Li,Rongqing" , Steven Rostedt , linux-kernel@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH] x86/math64: handle #DE in mul_u64_u64_div_u64() Message-ID: <20250724130037.1ae77797@pumpkin> In-Reply-To: <20250724082547.GB10980@redhat.com> References: <20250721130422.GA31640@redhat.com> <20250721192053.58843751@pumpkin> <20250722105034.GA2845@redhat.com> <20250722130947.0c97c96a@pumpkin> <20250722132147.GB2845@redhat.com> <20250722230341.5a1cc5e5@pumpkin> <20250723093825.GA12884@redhat.com> <20250723224831.4492ec75@pumpkin> <20250724081125.GA10980@redhat.com> <20250724082547.GB10980@redhat.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 24 Jul 2025 10:25:48 +0200 Oleg Nesterov wrote: > On 07/24, Oleg Nesterov wrote: > > On 07/23, David Laight wrote: > > > > > > On Wed, 23 Jul 2025 11:38:25 +0200 > > > Oleg Nesterov wrote: > > > > > > > > to remove the conditional branch and additional variable. Your version > > > > is probably beterr... But this is without WARN/BUG. > > > > > > I wish there was a way of doing a WARN_ONCE from asm with a single instruction. > > > Then you could put one after your 2: > > > Otherwise is it a conditional and a load of inlined code. > > > > > > > So, which version do you prefer? > > > > > > I wish I knew :-) > > > > ;-) > > > > David, you understand this asm magic indefinitely better than me. Plus you are > > working on the generic code. Can you send the patch which looks right to you? > > I agree in advance with anything you do. > > > > I got lost. Now I don't even understand if we want to add BUG and/or WARN into > > mul_u64_u64_div_u64(). > > Forgot to mention... Not that I think this is a good idea, but if we don't > use BUG/WARN, we can probably add EX_FLAG_ and do something like below. I'd not looked there. That is certainly best if WARN/BUG is deemed unnecessary. (That is the type of question I'd defer to 'management'!) > > Oleg. > > --- a/arch/x86/mm/extable.c > +++ b/arch/x86/mm/extable.c > @@ -38,6 +38,9 @@ static bool ex_handler_default(const struct exception_table_entry *e, > if (e->data & EX_FLAG_CLEAR_DX) > regs->dx = 0; > > + if (e->data & EX_FLAG_XXX_AX) > + regs->ax = -1ul; That would need to set %eax to a 64bit ~0u; I don't think the above would sign extend the value. Makes me think - always bad. I wonder how hard it would be to implement EX_FLAG_WARN_ONCE. Mostly it would need a writeable bitmap with one bit for each extable entry. David > + > regs->ip = ex_fixup_addr(e); > return true; > } >