From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received-SPF: Pass (sender SPF authorized) identity=mailfrom; client-ip=2607:f8b0:4864:20::d41; helo=mail-io1-xd41.google.com; envelope-from=ard.biesheuvel@linaro.org; receiver=edk2-devel@lists.01.org Received: from mail-io1-xd41.google.com (mail-io1-xd41.google.com [IPv6:2607:f8b0:4864:20::d41]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 65F0E2119BBE5 for ; Wed, 12 Dec 2018 04:45:30 -0800 (PST) Received: by mail-io1-xd41.google.com with SMTP id n3so3933279iog.12 for ; Wed, 12 Dec 2018 04:45:30 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=DFjsbZ99Ya+w/BF4fGKly1dKsoIAvCTWVIf4xoi6thU=; b=gu0YuzLz5M7GJyzZDsTLs3uUH/yS+LTFurw5dfQ9UPbXSciWsDRGyPs9NcY0k/DIKw apB5gfuGoOXFuwXUGCPmw4RW8ezhfb+gaf3WntqOzMMJQhRFIA6nB6uuZAS48sm2xzGy hrUbKdUyGzQwpFhRl0umTt3WQG7TeSSwtx9N0= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=DFjsbZ99Ya+w/BF4fGKly1dKsoIAvCTWVIf4xoi6thU=; b=GFUsTzQd5EMoC4uFw7GlloAKEFoRBgrKeNmNpAjpSqg/xjBoQNOd9Y4QqSpwlyEsTZ FIertHKxklsyK8XDeN6s8DCvmRobVhTLdxIx621F2HvOclpI3pIjVmmfX5SD4Axcscf/ y3I6VKW2IgC9O1bh877ObytQi/7pcJ7k90GJlbLwDBnOfELiEMVugCKBR0iMSRchitJl +8FKt39Fv0TkrHVKfsObe541ZMQ3/PaDu3528OdYvvuc8uPdLTtBiiYP2ImS+X/qZUAh mzryNPHx+VrdIA1LweN1uSdVBKNadLGmwADwCUjWjdSnKeHAC7JDe9+Ti5QAHYtr8Nqi Gc6g== X-Gm-Message-State: AA+aEWb6d1V5tZGDOp9sBsJMe0QHbGoVvacXd10xCjOyKBEDtVvKim87 nxEwUo5uJ46u+Zm3TwoEsb2JTIKmVi1SiPddgOkDNw== X-Google-Smtp-Source: AFSGD/Un+XIB79TuzzUpLvs8gU8MkqZdVI9kQ9jx8WyDM3wdDW+RG7JbCOHs40wnHGsaWsTH0Rh6+82oMlIXGls/EwI= X-Received: by 2002:a6b:5d01:: with SMTP id r1mr15655157iob.170.1544618729545; Wed, 12 Dec 2018 04:45:29 -0800 (PST) MIME-Version: 1.0 References: <20181212091211.13258-1-ard.biesheuvel@linaro.org> <20181212113009.vmsv37swaui3z4os@bivouac.eciton.net> <20181212123137.bzwi46gsszljlsvw@bivouac.eciton.net> In-Reply-To: <20181212123137.bzwi46gsszljlsvw@bivouac.eciton.net> From: Ard Biesheuvel Date: Wed, 12 Dec 2018 13:45:17 +0100 Message-ID: To: Leif Lindholm Cc: "edk2-devel@lists.01.org" Subject: Re: [PATCH] EmbeddedPkg/GdbDebugAgent ARM: use modern dialect for ldm/stm instructions X-BeenThere: edk2-devel@lists.01.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: EDK II Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 12 Dec 2018 12:45:31 -0000 Content-Type: text/plain; charset="UTF-8" On Wed, 12 Dec 2018 at 13:31, Leif Lindholm wrote: > > On Wed, Dec 12, 2018 at 12:33:07PM +0100, Ard Biesheuvel wrote: > > On Wed, 12 Dec 2018 at 12:30, Leif Lindholm wrote: > > > > > > On Wed, Dec 12, 2018 at 10:12:11AM +0100, Ard Biesheuvel wrote: > > > > Clang does not like the legacy 32-bit assembler syntax used in the > > > > gdbstub exception handling routines, so update them to something > > > > more fashionable. > > > > > > > > So switch the order of the conditional suffix > > > > > > Yes, this is essentially a bugfix. The syntax change predates ARM's > > > involvement in EDK2. > > > > > > > and the increment/decrement > > > > specifier, and use decrement-after (da) and increment-before (ib) as > > > > appropriate for an empty descending stack. > > > > > > But I very much prefer the FA/FD/EA/ED syntax. It gives symmetry, > > > which helps me a lot when dealing with 7-character mnemonics. > > > These aliases are also defined in the ARM ARM and won't be going away. > > > > Clang doesn't support them though. > > *Goes to check* > > *Boggles* > > Yeah, it supports FD/EA, but not FA/ED. That's ... special. > > > This code is not used anywhere (it is referenced in BeagleBoardPkg but > > commented out), so we could also simply remove it. > > > > > Contributed-under: TianoCore Contribution Agreement 1.1 > > > > Signed-off-by: Ard Biesheuvel > > > > --- > > > > EmbeddedPkg/Library/GdbDebugAgent/Arm/ExceptionSupport.ARMv6.S | 9 +++++---- > > > > 1 file changed, 5 insertions(+), 4 deletions(-) > > > > > > > > diff --git a/EmbeddedPkg/Library/GdbDebugAgent/Arm/ExceptionSupport.ARMv6.S b/EmbeddedPkg/Library/GdbDebugAgent/Arm/ExceptionSupport.ARMv6.S > > > > index df1543a6d242..924578c7af2f 100644 > > > > --- a/EmbeddedPkg/Library/GdbDebugAgent/Arm/ExceptionSupport.ARMv6.S > > > > +++ b/EmbeddedPkg/Library/GdbDebugAgent/Arm/ExceptionSupport.ARMv6.S > > > > @@ -57,6 +57,7 @@ GCC_ASM_EXPORT(AsmCommonExceptionEntry) > > > > GCC_ASM_EXPORT(GdbExceptionHandler) > > > > > > > > .text > > > > +.syntax unified > > > > .align 3 > > > > > > > > > > > > @@ -198,9 +199,9 @@ ASM_PFX(AsmCommonExceptionEntry): > > > > and R3, R1, #0x1f @ Check CPSR to see if User or System Mode > > > > cmp R3, #0x1f @ if ((CPSR == 0x10) || (CPSR == 0x1df)) > > > > cmpne R3, #0x10 @ > > > > - stmeqed R2, {lr}^ @ save unbanked lr > > > > + stmdaeq R2, {lr}^ @ save unbanked lr > > Then again, looking closer at these instructions, they're not really > doing stack operations. Just (ab)using the instruction to get at the > banked User mode LR from a different mode. So > a) The symmetry thing doesn't really apply, so the ED is actively > misleading here. > b) This could trivially be changed to use FD anyway, just setting R2's > offset from PC to #0x34 :) > Actually, since there is no writeback, what is the point of the decrement-after? Can't we just drop it? The more I look at this code the more broken it seems. I'll just remove it. > > > > @ else > > > > - stmneed R2, {lr} @ save SVC lr > > > > + stmdane R2, {lr} @ save SVC lr > > > > > > > > > > > > ldr R5, [SP, #0x58] @ PC is the LR pushed by srsfd > > > > @@ -245,9 +246,9 @@ GdbExceptionHandler ( > > > > and R1, R1, #0x1f @ Check to see if User or System Mode > > > > cmp R1, #0x1f @ if ((CPSR == 0x10) || (CPSR == 0x1f)) > > > > cmpne R1, #0x10 @ > > > > - ldmeqed R2, {lr}^ @ restore unbanked lr > > > > + ldmibeq R2, {lr}^ @ restore unbanked lr > > > > @ else > > > > - ldmneed R3, {lr} @ restore SVC lr, via ldmfd SP!, {LR} > > > > + ldmibne R3, {lr} @ restore SVC lr, via ldmfd SP!, {LR} > > > > > > > > ldmfd SP!,{R0-R12} @ Restore general purpose registers > > > > @ Exception handler can not change SP > > > > -- > > > > 2.19.2 > > > > > > c) But given a), I would take this one if the comment was updated to > be explicit about how esoteric this operation really is. > I.e.: "(ab)use STM^ to save banked User mode LR from SVC/HYP mode" > and "(ab)use LDM^ to restore banked User mode LR from SVC/HYP mode" > > I would also be happy to nuke it (including the commented out > inclusion for BeagleBoardPkg). > > / > Leif