From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40926) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dL7hQ-0008NO-OU for qemu-devel@nongnu.org; Wed, 14 Jun 2017 08:47:05 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dL7hN-0004Z6-Nv for qemu-devel@nongnu.org; Wed, 14 Jun 2017 08:47:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56782) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dL7hN-0004Yo-HM for qemu-devel@nongnu.org; Wed, 14 Jun 2017 08:47:01 -0400 References: <20170606181948.16238-1-rkagan@virtuozzo.com> <20170606181948.16238-17-rkagan@virtuozzo.com> <999e60c5-7ee7-5346-6940-1fcd5d7b4264@redhat.com> <20170614115421.GJ2004@rkaganb.sw.ru> <20170614124109.GB2044@rkaganb.sw.ru> From: Paolo Bonzini Message-ID: <7262a2b7-5c3c-39c6-8f2e-f2818ce59c35@redhat.com> Date: Wed, 14 Jun 2017 14:46:57 +0200 MIME-Version: 1.0 In-Reply-To: <20170614124109.GB2044@rkaganb.sw.ru> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 16/23] hyperv: map overlay pages after updating msrs List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Roman Kagan , qemu-devel@nongnu.org, Eduardo Habkost , Evgeny Yakovlev , "Denis V . Lunev" On 14/06/2017 14:41, Roman Kagan wrote: > On Wed, Jun 14, 2017 at 02:11:56PM +0200, Paolo Bonzini wrote: >> On 14/06/2017 13:54, Roman Kagan wrote: >>>> Why not disable the zeroing for host-initiated MSR writes? This is >>>> pretty clearly a KVM bug, we can push it to stable kernels too. >>> >>> The only problem with this is that QEMU will have no reliable way to >>> know if the KVM it runs with has this bug fixed or not. Machines >>> without vmbus work and even migrate fine with the current KVM despite >>> this bug (the only user of those pages currently is synic timers which >>> re-arm themselves and post messages regardless of zeroing). Now >>> updating QEMU to a vmbus-enabled version without updating the kernel >>> will make the migrations cause guest hangs. >> >> Return 2 from KVM_CHECK_EXTENSION(KVM_CAP_HYPERV_SYNIC)? Then you can >> make new QEMU refuse to enable synic if a new kernel is not available. > > Indeed, that's a possibility. > > I'll probably make it in both directions then: on > KVM_ENABLE_CAP(KVM_CAP_HYPERV_SYNIC, 2) disable zeroing completely, > including on guest writes, to better match Hyper-V. Or does it deserve > a separate cap number? Unfortunately kvm_vcpu_ioctl_enable_cap is not checking arguments and refusing nonzero values, so I'm a bit wary of doing that unconditionally---and I'm not sure it deserves a separate capability number. Maybe a flag KVM_ENABLE_CAP_STRICT_ARG_CHECK in cap->flags, but I don't want you to do too much unrelated work. Keeping the zeroing on guest writes doesn't seem too bad. Or the nuclear option, introduce KVM_CAP_HYPERV_SYNIC2 and drop the broken KVM_CAP_HYPERV_SYNIC altogether. Has its charm... Paolo