Thanks for following up, additional elements inline
Le 6 févr. 2024 à 10:37, Olivier Deprez Olivier.Deprez@arm.com a écrit :
Hi,
See my comment below [OD].
Regards, Olivier.
From: ff ff@shokubai.tech Sent: 05 February 2024 11:01 To: Olivier Deprez Olivier.Deprez@arm.com Cc: tf-a@lists.trustedfirmware.org tf-a@lists.trustedfirmware.org Subject: Re: MMIO with pre/post indexing instructions
Le 5 févr. 2024 à 10:38, Olivier Deprez Olivier.Deprez@arm.com a écrit :
Hi,
See few questions below [OD]
Regards, Olivier.
From: ff via TF-A tf-a@lists.trustedfirmware.org Sent: 04 February 2024 16:47 To: tf-a@lists.trustedfirmware.org tf-a@lists.trustedfirmware.org Subject: [TF-A] MMIO with pre/post indexing instructions Hi,
I am using hypervisors to allow execution of EL3 code directly
[OD] Can you elaborate a bit on the exact flow? IIUC 'hypervisor' implies running at EL2 so is this in context of calling an EL3 service from EL2 by SMC?
and had to deal with MMIO caused by instructions that have post-indexing parameters. Xen community had to deal with this in 2022: https://www.mail-archive.com/xen-devel@lists.xenproject.org/msg113335.html This does not cause any trouble and have implemented ways to deal with this expeditiously.
Yet, I wonder if it is desirable to from a synchronisation perspective?
[OD] If I understand properly, this is about Xen handling and recovering a data abort triggered by a VM. IIUC there is no such recovery logic at EL3. I have hard time understanding how the mentioned thread applies to TF-A's EL3 exception handling. Can you provide a bit more details?
Oh sorry for providing may be misleading information. The topic is really MMIO and what instructions the compiler generate when accessing those regions. For the same platform (Marvell macchiatobin) TFA compile options allows generation of post-indexing instructions to access MMIO regions while there none coming from Linux kernel (tested with 4.4 and 6.7). I am asking wether those instructions are OK to use for MMIO or not.
[OD] I believe I understand why this is done in context of Xen (EL2) handling and recovering a data abort originated from a VM (EL1). But I miss the use case as for why a similar 'workaround' would be required at EL3. AFAIK EL3 does not attempt recovering an data abort that would have occurred at a lower EL.
The hypervisor proper handling of MMIO is irrelevant to TF-A. If pre-post indexing instructions on MMIO area is legitimate and sound from a synchronization perspective, they just have to deal with this.
My very question is : « is using pre-post indexing instructions on MMIO OK to use? ».
To some extent it is not a TFA specific question either: it is an architectural question. But as I do not observe such instruction use in Kernel and there are plenty of Arm ISA experts contributing here, I just hope to get an authoritative answer here ;-)
Even if this was the case, the similar workaround would have to be done at EL2 and not EL3. Others on the mailing list may have differing opinions!
Cheers
FF
Compiling TFA 2.10 for PLAT=a80x0_mcbin
The following line
https://elixir.bootlin.com/arm-trusted-firmware/v2.10.0/source/drivers/marve...
Is compiled as:
win_reg = mmio_read_32(AMB_WIN_CR_OFFSET(win_id));4026d9c: f9470260 ldr x0, [x19, #3584] 4026da0: 9100e002 add x2, x0, #0x38 4026da4: b9400001 ldr w1, [x0] win_reg &= ~WIN_ENABLE_BIT; 4026da8: 121f7821 and w1, w1, #0xfffffffe *(volatile uint32_t*)addr = value; // post-indexing 4026dac: b8008401 str w1, [x0], #8
#gcc --version gcc (Ubuntu 10.3.0-1ubuntu1) 10.3.0 Copyright (C) 2020 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
#objdump --version GNU objdump (GNU Binutils for Ubuntu) 2.36.1 Copyright (C) 2021 Free Software Foundation, Inc. This program is free software; you may redistribute it under the terms of the GNU General Public License version 3 or (at your option) any later version. This program has absolutely no warranty.
#I can’t get access to latests compiler and binutils versions to overcome # « ../.. array subscript 0 is outside array bounds of ‘volatile void[0] » # « ../.. has a LOAD segment with RWX permissions » # so I don’t know if this behaviour is changed when using this latests toolchain
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