linux-kernel - Re: [PATCH v5 05/19] arm64: Add support for trace synchronization barrier

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Date:   Wed, 24 Mar 2021 17:06:58 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, mike.leach@...aro.org,
        leo.yan@...aro.org, anshuman.khandual@....com,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v5 05/19] arm64: Add support for trace synchronization
 barrier

On 24/03/2021 16:30, Marc Zyngier wrote:
> On Wed, 24 Mar 2021 16:25:12 +0000,
> Suzuki K Poulose <suzuki.poulose@....com> wrote:
>>
>> On 24/03/2021 16:16, Marc Zyngier wrote:
>>> On Wed, 24 Mar 2021 15:51:14 +0000,
>>> Suzuki K Poulose <suzuki.poulose@....com> wrote:
>>>>
>>>> On 24/03/2021 13:49, Marc Zyngier wrote:
>>>>> On Wed, 24 Mar 2021 09:39:13 +0000,
>>>>> Suzuki K Poulose <suzuki.poulose@....com> wrote:
>>>>>>
>>>>>> On 23/03/2021 18:21, Catalin Marinas wrote:
>>>>>>> Hi Suzuki?
>>>>>>>
>>>>>>> On Tue, Mar 23, 2021 at 12:06:33PM +0000, Suzuki K Poulose wrote:
>>>>>>>> tsb csync synchronizes the trace operation of instructions.
>>>>>>>> The instruction is a nop when FEAT_TRF is not implemented.
>>>>>>>>
>>>>>>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>>>>>>> Cc: Mike Leach <mike.leach@...aro.org>
>>>>>>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>>>>>>> Cc: Will Deacon <will.deacon@....com>
>>>>>>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>>>>>>
>>>>>>> How do you plan to merge these patches? If they go via the coresight
>>>>>>> tree:
>>>>>>>
>>>>>>
>>>>>> Ideally all of this should go via the CoreSight tree to have the
>>>>>> dependencies solved at one place. But there are some issues :
>>>>>>
>>>>>> If this makes to 5.13 queue for CoreSight,
>>>>>>
>>>>>> 1) CoreSight next is based on rc2 at the moment and we have fixes gone
>>>>>> into rc3 and later, which this series will depend on. (We could move
>>>>>> the next tree forward to a later rc to solve this).
>>>>>>
>>>>>> 2) There could be conflicts with the kvmarm tree for the KVM host
>>>>>> changes (That has dependency on the TRBE definitions patch).
>>>>>>
>>>>>> If it doesn't make to 5.13 queue, it would be good to have this patch,
>>>>>> the TRBE defintions and the KVM host patches queued for 5.13 (not sure
>>>>>> if this is acceptable) and we could rebase the CoreSight changes on 5.13
>>>>>> and push it to next release.
>>>>>>
>>>>>> I am open for other suggestions.
>>>>>>
>>>>>> Marc, Mathieu,
>>>>>>
>>>>>> Thoughts ?
>>>>>
>>>>> I was planning to take the first two patches in 5.12 as fixes (they
>>>>> are queued already, and would hopefully land in -rc5). If that doesn't
>>>>> fit with the plan, please let me know ASAP.
>>>>
>>>> Marc,
>>>>
>>>> I think it would be better to hold on pushing those patches until we
>>>> have a clarity on how things will go.
>>>
>>> OK. I thought there was a need for these patches to prevent guest
>>> access to the v8.4 self hosted tracing feature that went in 5.12
>>> though[1]... Did I get it wrong?
>>
>> Yes, that is correct. The guest could access the Trace Filter Control
>> register and fiddle with the host settings, without this patch.
>> e.g, it could disable tracing at EL0/EL1, without the host being
>> aware on nVHE host.
> 
> OK, so we definitely do need these patches, don't we? Both? Just one?
> Please have a look at kvmarm/fixes and tell me what I must keep.

Both of them are fixes.

commit "KVM: arm64: Disable guest access to trace filter controls"
  - This fixes guest fiddling with the trace filter control as described 
above.

commit "KVM: arm64: Hide system instruction access to Trace registers"
  - Fixes the Hypervisor to advertise what it doesn't support. i.e
    stop advertising trace system instruction access to a guest.
    Otherwise a guest which trusts the ID registers
    (ID_AA64DFR0_EL1.TRACEVER == 1) can crash while trying to access the
    trace register as we trap the accesses (CPTR_EL2.TTA == 1). On Linux,
    the ETM drivers need a DT explicitly advertising the support. So,
    this is not immediately impacted. And this fix goes a long way back
    in the history, when the CPTR_EL2.TTA was added.

Now, the reason for asking you to hold on is the way this could create
conflicts in merging the rest of the series.

Suzuki