From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH v8 15/17] x86/ctxt: Issue a speculation barrier between vcpu contexts Date: Thu, 18 Jan 2018 10:12:47 +0100 Message-ID: <1516266767.4937.118.camel@infradead.org> References: <1515780067-31735-1-git-send-email-andrew.cooper3@citrix.com> <1515780067-31735-16-git-send-email-andrew.cooper3@citrix.com> <1516020885.22147.152.camel@infradead.org> <3b1363be-ea08-80d0-6c90-c424c0f60dd7@citrix.com> <1516022599.22147.167.camel@infradead.org> <1516052372.4937.26.camel@infradead.org> <1516210004.4937.106.camel@infradead.org> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============3555526815015380389==" Return-path: In-Reply-To: <1516210004.4937.106.camel@infradead.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" To: Andrew Cooper , Xen-devel Cc: Matt Wilson List-Id: xen-devel@lists.xenproject.org --===============3555526815015380389== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-ysHRKm08eSgLsf1wlJy7" --=-ysHRKm08eSgLsf1wlJy7 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2018-01-17 at 18:26 +0100, David Woodhouse wrote: >=20 > > In both switching to idle, and back to the vCPU, we should hit this > > case and not the 'else' case that does the IBPB: > >=C2=A0 > > 1710=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if ( (per_cpu(curr_vcpu, cpu) =3D=3D = next) || > > 1711=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0(is_idl= e_domain(nextd) && cpu_online(cpu)) ) > > 1712=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0{ > > 1713=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0local_irq_ena= ble(); > > 1714=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} >=20 > I tested that; it doesn't seem to be the case. We end up here with prev > being the idle thread, next being the actual vCPU, and > per_cpu(curr_vcpu, cpu) being the idle thread too. So we still do the > IBPB even when we've just switch from a given vCPU to idle and back > again. >=20 > That's not actually tested on Xen master, but the code here looks very > much the same as what I actually did test. This appears to make the excessive IBPBs go away. There might be better approaches. diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c index 04e9902..b8a9d54 100644 --- a/xen/arch/x86/domain.c +++ b/xen/arch/x86/domain.c @@ -68,6 +68,7 @@ =C2=A0#include =C2=A0#include =C2=A0 +DEFINE_PER_CPU(struct vcpu *, last_vcpu); /* Last non-idle vCPU */ =C2=A0DEFINE_PER_CPU(struct vcpu *, curr_vcpu); =C2=A0 =C2=A0static void default_idle(void); @@ -1745,8 +1746,14 @@ void context_switch(struct vcpu *prev, struct vcpu *= next) =C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0ctxt_switch_levelling= (next); =C2=A0 -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if ( opt_ibpb ) +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/* IBPB on switching to a = non-idle vCPU, if that vCPU was not +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* the last one to be= scheduled on this pCPU */ +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if ( opt_ibpb && !is_idle_= cpu(next) && +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0per_cpu(last_vcpu, cpu) !=3D next ) +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0{ +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0pe= r_cpu(last_vcpu, cpu) =3D next; =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB); +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} =C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0context_saved(prev); --=-ysHRKm08eSgLsf1wlJy7 Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD 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