From patchwork Mon Dec 4 09:37:13 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Zijlstra X-Patchwork-Id: 13478004 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="UJ4QbxuB" Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E3DF8CD; Mon, 4 Dec 2023 01:39:52 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-Id:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=cKtSfZ5sWaSTj7tZINYGDLIwyP/HVC5Nz5I81xWPCwk=; b=UJ4QbxuBSELUeSrjN4lie1/3Nn sLbJRS71hZLDxuqqfx0RU270C87H+Ymw6e3gZXjYlGd71WgXV0HAq4BtolDaoiqcNJFkHY0dKMTG3 /zeZmKHZGL8lJchX5o+/rizS5A9SMrwY5iIjr8PSuKOAhtNFrTY22O9pFjYU1x33YdR0oVwCWAZjk mXcSzUX5aDu58oFsY3OITPf3xQLg3G6yydcoZcaTi0fHwDai0LrRHajnvymn3gIYbMh78WKnm2UFQ vECPvLWn/QORqhFYQA/SBQJQVoWs3DywzZmFe3jyjv0Kl/1ZZaUMHVjwGQAV5bdgJ92kTeaHa0F5n AgYZ1C6A==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1rA5QZ-004KSk-1O; Mon, 04 Dec 2023 09:39:47 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 0) id B32B7301C46; Mon, 4 Dec 2023 10:39:45 +0100 (CET) Message-Id: <20231204093732.580299853@infradead.org> User-Agent: quilt/0.65 Date: Mon, 04 Dec 2023 10:37:13 +0100 From: Peter Zijlstra To: Sean Christopherson , Paolo Bonzini , Josh Poimboeuf , Thomas Gleixner Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, x86@kernel.org, kvm@vger.kernel.org Subject: [PATCH 11/11] x86/kvm/emulate: Avoid RET for fastops References: <20231204093702.989848513@infradead.org> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Since there is only a single fastop() function, convert the FASTOP stuff from CALL_NOSPEC+RET to JMP_NOSPEC+JMP, avoiding the return thunks and all that jazz. Specifically FASTOPs rely on the return thunk to preserve EFLAGS, which not all of them can trivially do (call depth tracing suffers here). Objtool strenuously complains about this: - indirect call without a .rodata, fails to determine JUMP_TABLE, annotate - fastop functions fall through, exception - unreachable instruction after fastop_return, save/restore Signed-off-by: Peter Zijlstra (Intel) --- arch/x86/kvm/emulate.c | 20 +++++++++++++++----- include/linux/objtool_types.h | 1 + tools/include/linux/objtool_types.h | 1 + tools/objtool/check.c | 11 ++++++++++- 4 files changed, 27 insertions(+), 6 deletions(-) --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -285,8 +285,8 @@ static void invalidate_registers(struct * different operand sizes can be reached by calculation, rather than a jump * table (which would be bigger than the code). * - * The 16 byte alignment, considering 5 bytes for the RET thunk, 3 for ENDBR - * and 1 for the straight line speculation INT3, leaves 7 bytes for the + * The 16 byte alignment, considering 5 bytes for the JMP, 4 for ENDBR + * and 1 for the straight line speculation INT3, leaves 6 bytes for the * body of the function. Currently none is larger than 4. */ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop); @@ -304,7 +304,7 @@ static int fastop(struct x86_emulate_ctx __FOP_FUNC(#name) #define __FOP_RET(name) \ - "11: " ASM_RET \ + "11: jmp fastop_return; int3 \n\t" \ ".size " name ", .-" name "\n\t" #define FOP_RET(name) \ @@ -5071,14 +5071,24 @@ static void fetch_possible_mmx_operand(s kvm_read_mmx_reg(op->addr.mm, &op->mm_val); } -static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop) +/* + * All the FASTOP magic above relies on there being *one* instance of this + * so it can JMP back, avoiding RET and it's various thunks. + */ +static noinline int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop) { ulong flags = (ctxt->eflags & EFLAGS_MASK) | X86_EFLAGS_IF; if (!(ctxt->d & ByteOp)) fop += __ffs(ctxt->dst.bytes) * FASTOP_SIZE; - asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n" + asm("push %[flags]; popf \n\t" + UNWIND_HINT(UNWIND_HINT_TYPE_SAVE, 0, 0, 0) + ASM_ANNOTATE(ANNOTYPE_JUMP_TABLE) + JMP_NOSPEC + "fastop_return: \n\t" + UNWIND_HINT(UNWIND_HINT_TYPE_RESTORE, 0, 0, 0) + "pushf; pop %[flags]\n" : "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags), [thunk_target]"+S"(fop), ASM_CALL_CONSTRAINT : "c"(ctxt->src2.val)); --- a/include/linux/objtool_types.h +++ b/include/linux/objtool_types.h @@ -64,5 +64,6 @@ struct unwind_hint { #define ANNOTYPE_UNRET_BEGIN 5 #define ANNOTYPE_IGNORE_ALTS 6 #define ANNOTYPE_INTRA_FUNCTION_CALLS 7 +#define ANNOTYPE_JUMP_TABLE 8 #endif /* _LINUX_OBJTOOL_TYPES_H */ --- a/tools/include/linux/objtool_types.h +++ b/tools/include/linux/objtool_types.h @@ -64,5 +64,6 @@ struct unwind_hint { #define ANNOTYPE_UNRET_BEGIN 5 #define ANNOTYPE_IGNORE_ALTS 6 #define ANNOTYPE_INTRA_FUNCTION_CALLS 7 +#define ANNOTYPE_JUMP_TABLE 8 #endif /* _LINUX_OBJTOOL_TYPES_H */ --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -2386,6 +2386,14 @@ static int __annotate_late(struct objtoo insn->unret = 1; break; + /* + * Must be after add_jump_table(); for it doesn't set a sane + * _jump_table value. + */ + case ANNOTYPE_JUMP_TABLE: + insn->_jump_table = (void *)1; + break; + default: break; } @@ -3459,7 +3467,8 @@ static int validate_branch(struct objtoo if (func && insn_func(insn) && func != insn_func(insn)->pfunc) { /* Ignore KCFI type preambles, which always fall through */ if (!strncmp(func->name, "__cfi_", 6) || - !strncmp(func->name, "__pfx_", 6)) + !strncmp(func->name, "__pfx_", 6) || + !strcmp(insn_func(insn)->name, "fastop")) return 0; WARN("%s() falls through to next function %s()",