From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH] kvm: First step to push iothread lock out of inner run loop Date: Sat, 23 Jun 2012 13:45:57 +0200 Message-ID: <4FE5AC75.1020504@web.de> References: <4FE4F56D.1020201@web.de> <4FE4F7F5.7030400@web.de> <20120623002259.GA13440@amt.cnet> <20120623090646.GA21908@amt.cnet> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigA91B920DCC584A994854F53C" Cc: qemu-devel , Liu Ping Fan , Alexander Graf , Anthony Liguori , kvm , Avi Kivity To: Marcelo Tosatti Return-path: Received: from mout.web.de ([212.227.17.11]:51075 "EHLO mout.web.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751088Ab2FWLqF (ORCPT ); Sat, 23 Jun 2012 07:46:05 -0400 In-Reply-To: <20120623090646.GA21908@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigA91B920DCC584A994854F53C Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On 2012-06-23 11:06, Marcelo Tosatti wrote: > On Fri, Jun 22, 2012 at 09:22:59PM -0300, Marcelo Tosatti wrote: >> On Sat, Jun 23, 2012 at 12:55:49AM +0200, Jan Kiszka wrote: >>> Should have declared this [RFC] in the subject and CC'ed kvm... >>> >>> On 2012-06-23 00:45, Jan Kiszka wrote: >>>> This sketches a possible path to get rid of the iothread lock on vme= xits >>>> in KVM mode. On x86, the the in-kernel irqchips has to be used becau= se >>>> we otherwise need to synchronize APIC and other per-cpu state access= es >>>> that could be changed concurrently. Not yet fully analyzed is the NM= I >>>> injection path in the absence of an APIC. >>>> >>>> s390x should be fine without specific locking as their pre/post-run >>>> callbacks are empty. Power requires locking for the pre-run callback= =2E >>>> >>>> This patch is untested, but a similar version was successfully used = in >>>> a x86 setup with a network I/O path that needed no central iothread >>>> locking anymore (required special MMIO exit handling). >>>> --- >>>> kvm-all.c | 18 ++++++++++++++++-- >>>> target-i386/kvm.c | 7 +++++++ >>>> target-ppc/kvm.c | 4 ++++ >>>> 3 files changed, 27 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/kvm-all.c b/kvm-all.c >>>> index f8e4328..9c3e26f 100644 >>>> --- a/kvm-all.c >>>> +++ b/kvm-all.c >>>> @@ -1460,6 +1460,8 @@ int kvm_cpu_exec(CPUArchState *env) >>>> return EXCP_HLT; >>>> } >>>> =20 >>>> + qemu_mutex_unlock_iothread(); >>>> + >>>> do { >>>> if (env->kvm_vcpu_dirty) { >>>> kvm_arch_put_registers(env, KVM_PUT_RUNTIME_STATE); >>>> @@ -1476,14 +1478,16 @@ int kvm_cpu_exec(CPUArchState *env) >>>> */ >>>> qemu_cpu_kick_self(); >>>> } >>>> - qemu_mutex_unlock_iothread(); >>>> =20 >>>> run_ret =3D kvm_vcpu_ioctl(env, KVM_RUN, 0); >>>> =20 >>>> - qemu_mutex_lock_iothread(); >>>> kvm_arch_post_run(env, run); >> >> target-i386/kvm.c >> >> void kvm_arch_post_run(CPUX86State *env, struct kvm_run *run) >> { =20 >> if (run->if_flag) { >> env->eflags |=3D IF_MASK; >> } else { >> env->eflags &=3D ~IF_MASK; >> } >> cpu_set_apic_tpr(env->apic_state, run->cr8); >> cpu_set_apic_base(env->apic_state, run->apic_base); >> } >> >> Clearly there is no structure to any of the writes around the writes >> in x86's kvm_arch_post_run, so it is unsafe. >=20 > No access protocol to the CPUState and apic devices (who can write when= , > who can read when). >=20 Hmm, we may need the iothread lock around cpu_set_apic_tpr for !kvm_irqchip_in_kernel(). And as we are at it, apic_base manipulation can be but there as well. With in-kernel irqchip, there is no such need. Also, no one accesses eflags outside of the vcpu thread, independent of the irqchip mode. Jan --------------enigA91B920DCC584A994854F53C Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.16 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk/lrHgACgkQitSsb3rl5xQhIQCeNR+G1WMwUUtrQESl7ta4bDu+ nLMAn0Gb9F3Ht/GAgjn//9D+lBVJ2Q12 =3b1r -----END PGP SIGNATURE----- --------------enigA91B920DCC584A994854F53C--