Re: [PATCH v9 09/29] arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE

Steven Price
2020-09-10T12:45:02Z

On 10/09/2020 12:55, Catalin Marinas wrote:
> On Thu, Sep 10, 2020 at 12:12:27PM +0100, Steven Price wrote:
>> On 10/09/2020 11:52, Catalin Marinas wrote:
>>> On Thu, Sep 10, 2020 at 11:23:33AM +0100, Steven Price wrote:
>>>> On 04/09/2020 11:30, Catalin Marinas wrote:
>>>>> --- /dev/null
>>>>> +++ b/arch/arm64/lib/mte.S
>>>>> @@ -0,0 +1,34 @@
>>>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>>>> +/*
>>>>> + * Copyright (C) 2020 ARM Ltd.
>>>>> + */
>>>>> +#include <linux/linkage.h>
>>>>> +
>>>>> +#include <asm/assembler.h>
>>>>> +#include <asm/sysreg.h>
>>>>> +
>>>>> + .arch armv8.5-a+memtag
>>>>> +
>>>>> +/*
>>>>> + * multitag_transfer_size - set \reg to the block size that is accessed by the
>>>>> + * LDGM/STGM instructions.
>>>>> + */
>>>>> + .macro multitag_transfer_size, reg, tmp
>>>>> + mrs_s \reg, SYS_GMID_EL1
>>>>> + ubfx \reg, \reg, #SYS_GMID_EL1_BS_SHIFT, #SYS_GMID_EL1_BS_SIZE
>>>>> + mov \tmp, #4
>>>>> + lsl \reg, \tmp, \reg
>>>>> + .endm
>>>>> +
>>>>> +/*
>>>>> + * Clear the tags in a page
>>>>> + * x0 - address of the page to be cleared
>>>>> + */
>>>>> +SYM_FUNC_START(mte_clear_page_tags)
>>>>> + multitag_transfer_size x1, x2
>>>>> +1: stgm xzr, [x0]
>>>>> + add x0, x0, x1
>>>>> + tst x0, #(PAGE_SIZE - 1)
>>>>> + b.ne 1b
>>>>> + ret
>>>>> +SYM_FUNC_END(mte_clear_page_tags)
>>>>
>>>> Could the value of SYS_GMID_EL1 vary between CPUs and do we therefore need a
>>>> preempt_disable() around mte_clear_page_tags() (and other functions in later
>>>> patches)?
>>>
>>> If they differ, disabling preemption here is not sufficient. We'd have
>>> to trap the GMID_EL1 access at EL2 as well and emulate it (we do this
>>> for CTR_EL0 in dcache_line_size).
>>
>> Hmm, good point. It's actually not possible to properly emulate this - EL2
>> can trap GMID_EL1 to provide a different (presumably smaller) size, but
>> LDGM/STGM will still read/store the number of tags of the underlying
>> hardware. While simple loops like we've got at the moment won't care (we'll
>> just end up doing useless work), it won't be architecturally correct. The
>> guest can always deduce the underlying value. So I think we can safely
>> consider this broken hardware.
>
> I think that's similar to the DC ZVA (and DCZID_EL0.BS) case where
> faking it could lead to data corruption if the software assumes it
> writes a maximum number of bytes.
>
> (I meant to raise a ticket with the architects to make this a
> requirement in the ARM ARM but forgot about it)

Yes, that looks like exactly the same issue.

>>> I don't want to proactively implement this just in case we'll have
>>> broken hardware (I feel a bit more optimistic today ;)).
>>
>> Given the above I think if we do have broken hardware the only sane thing to
>> do would be to provide a way of overriding multitag_transfer_size to return
>> the smallest size of all CPUs. Which works well enough for the uses we've
>> currently got.
>
> If we do have such broken hardware, we should probably drop the STGM
> instructions in favour of STG or ST2G. Luckily, STGM/LDGM are not
> available in user space.
>

STGM should be safe the way we're doing it as long as the block size
we're using is <= the hardware block size (it'll just write multiple
times, but we're writing more than the maximum block size so there's no
data loss). Although it would be worthwhile to benchmark on the hardware
to see whether it's actually worth it in that case or if STG/ST2G is
actually faster.

Steve