From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44431) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gWpKT-0004PN-6P for qemu-devel@nongnu.org; Tue, 11 Dec 2018 16:12:34 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gWpKP-00068N-9V for qemu-devel@nongnu.org; Tue, 11 Dec 2018 16:12:33 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:54182) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gWpKO-00062F-33 for qemu-devel@nongnu.org; Tue, 11 Dec 2018 16:12:29 -0500 Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wBBL8m4h054913 for ; Tue, 11 Dec 2018 16:12:22 -0500 Received: from e35.co.us.ibm.com (e35.co.us.ibm.com [32.97.110.153]) by mx0a-001b2d01.pphosted.com with ESMTP id 2pajuydvhy-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 11 Dec 2018 16:12:22 -0500 Received: from localhost by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 11 Dec 2018 21:12:21 -0000 From: Collin Walling References: <1544135058-21380-1-git-send-email-walling@linux.ibm.com> <1544135058-21380-3-git-send-email-walling@linux.ibm.com> <20181207130853.20506345.cohuck@redhat.com> Date: Tue, 11 Dec 2018 16:12:16 -0500 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: Subject: Re: [Qemu-devel] [qemu-s390x] [PATCH v2 2/3] s390: cpu feature for diagnose 318 andlimit max VCPUs to 247 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck Cc: thuth@redhat.com, david@redhat.com, qemu-devel@nongnu.org, borntraeger@de.ibm.com, qemu-s390x@nongnu.org, rth@twiddle.net On 12/11/18 11:47 AM, Collin Walling wrote: > On 12/7/18 7:08 AM, Cornelia Huck wrote: >> On Thu, 6 Dec 2018 17:24:17 -0500 >> Collin Walling wrote: >> >>> Diagnose 318 is a new z14.2 CPU feature. Since we are able to emulate >>> it entirely via KVM, we can add guest support for earlier models. A >>> new CPU feature for diagnose 318 (shortened to diag318) will be made >>> available to guests starting with the zEC12-full CPU model. >>> >>> The z14.2 adds a new read SCP info byte (let's call it byte 134) to >>> detect the availability of diag318. Because of this, we have room for >>> one less VCPU and thus limit the max VPUs supported in a configuration >>> to 247 (down from 248). >>> >>> Signed-off-by: Collin Walling . >>> --- >>> hw/s390x/sclp.c | 2 ++ >>> include/hw/s390x/sclp.h | 2 ++ >>> target/s390x/cpu.h | 2 +- >>> target/s390x/cpu_features.c | 3 +++ >>> target/s390x/cpu_features.h | 1 + >>> target/s390x/cpu_features_def.h | 3 +++ >>> target/s390x/gen-features.c | 1 + >>> target/s390x/kvm.c | 1 + >>> 8 files changed, 14 insertions(+), 1 deletion(-) >>> >> >>> diff --git a/target/s390x/cpu.h b/target/s390x/cpu.h >>> index 8c2320e..594b4a4 100644 >>> --- a/target/s390x/cpu.h >>> +++ b/target/s390x/cpu.h >>> @@ -52,7 +52,7 @@ >>> >>> #define MMU_USER_IDX 0 >>> >>> -#define S390_MAX_CPUS 248 >>> +#define S390_MAX_CPUS 247 >> >> Isn't that already problematic if you try to migrate from an older QEMU >> with all possible vcpus defined? IOW, don't you really need a way that >> older machines can still run with one more vcpu? >> > > Good call. I'll run some tests on this and see what happens. I'll report > here on those results. > Migrating to a machine that supports less vCPUs will report error: unsupported configuration: Maximum CPUs greater than specified machine type limit I revisited the code to see if there's a way to dynamically set the max vcpu count based on the read scp info size, but it gets really tricky and code looks very complicated. (Having a packed struct contain the CPU entries whose maximum is determined by hardware limitations makes things difficult -- but who said s390 is easy? :) ) In reality, do we often have guests running with 248 or even 247 vcpus? If so, I imagine the performance isn't too significant? >>> >>> typedef struct PSW { >>> uint64_t mask; >> > > -- Respectfully, - Collin Walling