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: Wed, 17 Jan 2018 18:26:44 +0100 Message-ID: <1516210004.4937.106.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> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1737976116594043944==" Return-path: In-Reply-To: <1516052372.4937.26.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 --===============1737976116594043944== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-/mYTnxFpKjRTaEwvYIzZ" --=-/mYTnxFpKjRTaEwvYIzZ Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-15 at 22:39 +0100, David Woodhouse wrote: > On Mon, 2018-01-15 at 14:23 +0100, David Woodhouse wrote: > >=20 > >=20 > > >=20 > > > >=20 > > > > =C2=A0 > > > > Also... if you're doing that in context_switch() does it do the rig= ht > > > > thing with idle? If a CPU switches to the idle domain and then back > > > > again to the same vCPU, does that do the IBPB twice? > > > > > > Context switches to idle will skip the IBPB because it isn't needed, = but > > > any switch to non-idle need it.=C2=A0 In your example, we should exec= ute just > > > a single IBPB. > > > In my example I think we should not execute IBPB at all. We come from a > > given VMCS, sleep for a while, and go back to it. No need for any > > flushing whatsoever. > > msw points out that in my example we *don't* execute IBPB at all, I > think. >=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: >=20 > 1710=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if ( (per_cpu(curr_vcpu, cpu) =3D=3D ne= xt) || > 1711=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0(is_idle_= 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_enabl= e(); > 1714=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0} 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. That's not actually tested on Xen master, but the code here looks very much the same as what I actually did test. --=-/mYTnxFpKjRTaEwvYIzZ Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu dGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQTAeFw0xNzEyMjEwMDAwMDBaFw0xODEyMjEyMzU5 NTlaMCQxIjAgBgkqhkiG9w0BCQEWE2R3bXcyQGluZnJhZGVhZC5vcmcwggEiMA0GCSqGSIb3DQEB AQUAA4IBDwAwggEKAoIBAQDgzLNWa18DNpGUj/ZeH0Sgz53ESIbzdPw3OJeuNP6jZhxZojbyfxbM hETscxI/Hj6UZ4a7sHm5BkVjlsB1Af2Za/PXUt8MmLAcPMHkMPGunvkUibEvblDvpqMkQZlaZM+t 5PqFmWkbehLaEvbpNY7dmEAAeKh4klTzJzrr5AAzaCQ32cA2e3+DEIv5O5l9ViMIjy/JM+xMQrfX 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