From: Peter Zijlstra <peterz@infradead.org>
To: Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
x86@kernel.org, kvm@vger.kernel.org
Subject: [PATCH 00/11] x86/kvm/emulate: Avoid RET for FASTOPs
Date: Mon, 04 Dec 2023 10:37:02 +0100 [thread overview]
Message-ID: <20231204093702.989848513@infradead.org> (raw)
Hi!
Because I needed a new objtool annotation, and I'd promised Josh I'd clean all
that up a while ago, now with a healy sprinking of objtool patches...
Anyway... FASTOP is special in that it relies on RET to preserve FLAGS, while
normal C calling convention does not. This has been a problem before, see
ba5ca5e5e6a1 ("x86/retpoline: Don't clobber RFLAGS during srso_safe_ret()") but
is also a problem for call depth tracking.
Fixing the call-depth tracking return thunk would be significantly harder (and
more expensive), so instead change fastops to not use return.
There are two separate instances, test_cc() and fastop(). The first is
basically a SETCC wrapper, which seems like a very complicated (and somewhat
expensive) way to read FLAGS. Instead use the code we already have to emulate
JCC to fully emulate the instruction.
That then leaves fastop(), which when marked noinline is guaranteed to exist
only once. As such, CALL+RET isn't needed, because we'll always be RETurning to
the same location, as such replace with JMP+JMP.
---
arch/x86/include/asm/alternative.h | 14 +-
arch/x86/include/asm/nospec-branch.h | 45 ++++--
arch/x86/include/asm/text-patching.h | 20 ++-
arch/x86/kvm/emulate.c | 54 +++----
include/linux/instrumentation.h | 11 +-
include/linux/objtool.h | 62 +++++----
include/linux/objtool_types.h | 12 ++
tools/include/linux/objtool_types.h | 12 ++
tools/objtool/check.c | 263 +++++++++++------------------------
9 files changed, 208 insertions(+), 285 deletions(-)
next reply other threads:[~2023-12-04 9:40 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-04 9:37 Peter Zijlstra [this message]
2023-12-04 9:37 ` [PATCH 01/11] objtool: Generic annotation infrastructure Peter Zijlstra
2023-12-04 9:37 ` [PATCH 02/11] objtool: Convert ANNOTATE_NOENDBR to ANNOTATE Peter Zijlstra
2023-12-04 9:37 ` [PATCH 03/11] objtool: Convert ANNOTATE_RETPOLINE_SAFE " Peter Zijlstra
2023-12-06 23:31 ` Sean Christopherson
2023-12-04 9:37 ` [PATCH 04/11] objtool: Convert instrumentation_{begin,end}() " Peter Zijlstra
2023-12-04 9:37 ` [PATCH 05/11] objtool: Convert VALIDATE_UNRET_BEGIN " Peter Zijlstra
2023-12-04 9:37 ` [PATCH 06/11] objtool: Convert ANNOTATE_IGNORE_ALTERNATIVE " Peter Zijlstra
2023-12-04 9:37 ` [PATCH 07/11] objtool: Convert ANNOTATE_INTRA_FUNCTION_CALLS " Peter Zijlstra
2023-12-04 9:37 ` [PATCH 08/11] objtool: Collapse annotate sequences Peter Zijlstra
2023-12-04 9:37 ` [PATCH 09/11] x86/kvm/emulate: Implement test_cc() in C Peter Zijlstra
2023-12-04 9:37 ` [PATCH 10/11] x86/nospec: JMP_NOSPEC Peter Zijlstra
2023-12-04 9:37 ` [PATCH 11/11] x86/kvm/emulate: Avoid RET for fastops Peter Zijlstra
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