From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH v6.5 11/26] x86: Support indirect thunks from assembly code Date: Thu, 11 Jan 2018 13:46:55 +0000 Message-ID: <1515678415.22302.342.camel@infradead.org> References: <1515024955-13390-1-git-send-email-andrew.cooper3@citrix.com> <1515024955-13390-12-git-send-email-andrew.cooper3@citrix.com> <1515675795.22302.336.camel@infradead.org> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============7481237242142131133==" Return-path: In-Reply-To: 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 --===============7481237242142131133== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-Dc5QjBKevg4XX10tunEq" --=-Dc5QjBKevg4XX10tunEq Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2018-01-11 at 13:41 +0000, Andrew Cooper wrote: > On 11/01/18 13:03, David Woodhouse wrote: > >=20 > > On Thu, 2018-01-04 at 00:15 +0000, Andrew Cooper wrote: > > >=20 > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* We've got no= usable stack so can't use a RETPOLINE thunk, and are > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* further than= +- 2G from the high mappings so couldn't use JUMP_THUNK > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* even if was = a non-RETPOLINE thunk.=C2=A0=C2=A0Futhermore, an LFENCE isn't > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0* necesserily = safe to use at this point. > > > I count three typos, pedantry about =C2=B1 and GiB aside. > > Late night? :) > Just one of many... >=20 > I've found furthermore and necessarily.=C2=A0 Where is the 3rd? * even if IT was a =E2=80=A6=C2=A0 > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0asm volatile ( "call= *%[stb]\n" > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0asm volatile ( "CALL= _THUNK %[stb]\n" > > If you make that %V[stb] then... > > ... you don't need this. > That's fine in principle, except it isn't compatible with most of the > compilers we support.=C2=A0 To use, the %V has to be hidden behind a > conditional macro, and I can't think of any remotely-clean way to do > that. In at least one incarnation I ended up with something like #ifndef CONFIG_RETPOLINE #define CALL_THUNK(reg) "call *%[" #reg "]" #else #define CALL_THUNK(reg) "CALL_THUNK %V[" #reg "]" #endif Or maybe I just insisted that it was called %[thunk_target] and my CALL_THUNK C macro doesn't even take an argument. I forget. 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