From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sasha Levin Subject: Re: Fix for bug that causes "KVM_GET_SUPPORTED_CPUID failed" errors. Date: Tue, 24 Jan 2012 02:40:04 -0500 Message-ID: <1327390804.32498.2.camel@lappy> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm , Ronald Minnich To: Gabe Black Return-path: Received: from mail-vx0-f174.google.com ([209.85.220.174]:46139 "EHLO mail-vx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751464Ab2AXFkf (ORCPT ); Tue, 24 Jan 2012 00:40:35 -0500 Received: by vcbgb30 with SMTP id gb30so1542829vcb.19 for ; Mon, 23 Jan 2012 21:40:34 -0800 (PST) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: The GET_SUPPORTED_CPUID bug has been fixed and shouldn't be happening from v3.2 onwards. Do you still see the issue in older versions? On Mon, 2012-01-23 at 21:20 -0800, Gabe Black wrote: > Sorry, forgot to add a subject. > > Gabe > > On Mon, Jan 23, 2012 at 9:18 PM, Gabe Black wrote: > > Hi, I think I've tracked down the bug that causes > > "KVM_GET_SUPPORTED_CPUID failed: Argument list too long" errors when > > using the kvm tool. Basically, this (possibly squished) code seems to > > be to blame: > > > > case 0xd: { > > int i; > > > > entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > > for (i = 1; *nent < maxnent && i < 64; ++i) { > > if (entry[i].eax == 0) > > continue; > > do_cpuid_1_ent(&entry[i], function, i); > > entry[i].flags |= > > KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > > ++*nent; > > } > > break; > > } > > > > You can see there's a check whether entry[i].eax is 0, but it isn't > > until the next line that entry[i] is actually filled in. That means > > that whether or not an entry is filled in for the 0xd function is > > essentially random, and that can lead to the loss of valid entries. It > > also means that nent may be incremented too often, and since all 64 > > entries are iterated over, that can fill up the available storage and > > cause that error. > > > > I tested my theory by commenting out the if (100% failure rate) and > > moving it after do_cpuid_1_ent (100% success rate). Since this is a > > non-deterministic failure that isn't really conclusive, but I'm fairly > > confident my fix is correct. I don't know exactly what your procedure > > is for submitting patches, but one is attached. > > > > Gabe > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Sasha.