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: Mon, 15 Jan 2018 14:23:19 +0100 Message-ID: <1516022599.22147.167.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> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1250611605951757135==" Return-path: In-Reply-To: <3b1363be-ea08-80d0-6c90-c424c0f60dd7@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" To: Andrew Cooper , Xen-devel List-Id: xen-devel@lists.xenproject.org --===============1250611605951757135== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-5eRIyJVwuRY0HBuuyGiw" --=-5eRIyJVwuRY0HBuuyGiw Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-15 at 13:02 +0000, Andrew Cooper wrote: > On 15/01/18 12:54, David Woodhouse wrote: > >=20 > > On Fri, 2018-01-12 at 18:01 +0000, Andrew Cooper wrote: > > >=20 > > > @@ -1736,6 +1736,9 @@ 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=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ctxt_switch_levellin= g(next); > > > + > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if ( opt_ibpb ) > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 w= rmsrl(MSR_PRED_CMD, PRED_CMD_IBPB); > > > =C2=A0=C2=A0=C2=A0=C2=A0 } > > >=20 > > If you're doing that without an 'else lfence' don't you need to include > > a comment with your proof that it's safe to do so, and the CPU can't > > speculate a taken conditional branch and all the way to a problematic > > instruction? > > What would that gain?=C2=A0 A malicious guest could speculate around it, = but > speculation will catch up (at the very latest) when we return to guest, > which is a serialising event. There's your proof. I'd just be happier with a blanket policy of *including* that proof in all cases where we do this kind of runtime conditional branch around setting IBRS or IBPB. Because if we're in the habit of doing the 'if (foo) wrmsrl()' without it, we *might* miss a case where it's not actually OK. (Aside: Is VMLAUNCH actually architecturally guaranteed to be serialising? That doesn't seem consistent with a long-term goal of designing hardware to make VMs go faster. Or have we merely extracted a promise from Intel that *current* hardware will stop speculative execution when it hits a VMLAUNCH?) =C2=A0 > >=20 > > Also... if you're doing that in context_switch() does it do the right > > 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 execute = 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. > >=20 > > For vmx we only really need IBPB when we do VMPTRLD, right? > > That is part of IBRS_ATT is it not? It doesn't go away with IBRS_ALL (as it's now called), but it's the same for the existing IBRS *and* retpoline. Doing it on VMPTRLD is what Linux is doing. (cf.=C2=A0https://lkml.org/lkml/2018/1/13/40=C2=A0and =C2=A0 =C2=A0 =C2=A0https://lkml.org/lkml/2018/1/15/146=C2=A0and note the A= MD SVM caveat.) --=-5eRIyJVwuRY0HBuuyGiw 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 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