From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation Date: Sun, 21 Jan 2018 22:00:13 +0000 Message-ID: <1516572013.9814.109.camel@infradead.org> References: <1516476182-5153-1-git-send-email-karahmed@amazon.de> <1516476182-5153-10-git-send-email-karahmed@amazon.de> <1516566497.9814.78.camel@infradead.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-njR401xZhB+xTgdAg6mZ" Cc: KarimAllah Ahmed , Linux Kernel Mailing List , Andi Kleen , Andrea Arcangeli , Andy Lutomirski , Arjan van de Ven , Ashok Raj , Asit Mallick , Borislav Petkov , Dan Williams , Dave Hansen , Greg Kroah-Hartman , "H . Peter Anvin" , Ingo Molnar , Janakarajan Natarajan , Joerg Roedel , Jun Nakajima , Laura Abbott , Masami Hiramatsu , Paolo To: Linus Torvalds Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org --=-njR401xZhB+xTgdAg6mZ Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Sun, 2018-01-21 at 13:35 -0800, Linus Torvalds wrote: > On Sun, Jan 21, 2018 at 12:28 PM, David Woodhouse wrote: > > As a hack for existing CPUs, it's just about tolerable =E2=80=94 as lon= g as it > > can die entirely by the next generation. > > That's part of the big problem here. The speculation control cpuid > stuff shows that Intel actually seems to plan on doing the right thing > for meltdown (the main question being _when_). Which is not a huge > surprise, since it should be easy to fix, and it's a really honking > big hole to drive through. Not doing the right thing for meltdown > would be completely unacceptable. >=20 > So the IBRS garbage implies that Intel is _not_ planning on doing the > right thing for the indirect branch speculation. >=20 > Honestly, that's completely unacceptable too. Agreed. I've been saying that since I first saw the IBRS_ALL proposal. There's *no* good reason for it to be opt-in. Just fix it! > > So the part is I think is odd is the IBRS_ALL feature, where a future > > CPU will advertise "I am able to be not broken" and then you have to > > set the IBRS bit once at boot time to *ask* it not to be broken. That > > part is weird, because it ought to have been treated like the RDCL_NO > > bit =E2=80=94 just "you don't have to worry any more, it got better". > > It's not "weird" at all. It's very much part of the whole "this is > complete garbage" issue. >=20 > The whole IBRS_ALL feature to me very clearly says "Intel is not > serious about this, we'll have a ugly hack that will be so expensive > that we don't want to enable it by default, because that would look > bad in benchmarks". >=20 > So instead they try to push the garbage down to us. And they are doing > it entirely wrong, even from a technical standpoint. Right. The whole IBRS/IBPB thing as a nasty hack in the short term I could live with, but it's the long-term implications of IBRS_ALL that I'm unhappy about. My understanding was that the IBRS_ALL performance was supposed to not suck =E2=80=94 to the extent that we'd just turn it on and then ALTERNATIVE= out the retpolines, and that would be the best option. But if that's the case, why are they making it an option, and not just doing the same as RDCL_NO does for "we fixed Meltdown"? > > We do need the IBPB feature to complete the protection that retpoline > > gives us =E2=80=94 it's that or rebuild all of userspace with retpoline= . > > BULLSHIT. >=20 > Have you _looked_ at the patches you are talking about?=C2=A0=C2=A0You sh= ould > have - several of them bear your name. >=20 > The patches do things like add the garbage MSR writes to the kernel > entry/exit points. That's insane. That says "we're trying to protect > the kernel".=C2=A0=C2=A0We already have retpoline there, with less overhe= ad. You're looking at IBRS usage, not IBPB. They are different things. Yes, the one you're looking at really *is* trying to protect the kernel, and you're right that it's largely redundant with retpoline. (Assuming we can live with the implications on Skylake, as I said.) > If this was about flushing the BTB at actual context switches between > different users, I'd believe you. But that's not at all what the > patches do. That's what the *IBPB* patches do. 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