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

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Date:   Fri, 26 Mar 2021 10:31:39 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        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 Wed, Mar 24, 2021 at 05:40:39PM +0000, Marc Zyngier wrote:
> On Wed, 24 Mar 2021 17:19:36 +0000,
> Catalin Marinas <catalin.marinas@....com> wrote:
> > 
> > On Wed, Mar 24, 2021 at 05:06:58PM +0000, Suzuki K Poulose wrote:
> > > On 24/03/2021 16:30, Marc Zyngier wrote:
> > > >
> > > > 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.
> > 
> > The way we normally work around this is to either rebase your series on
> > top of -rc5 when the fixes go in or, if you want an earlier -rc base,
> > Marc can put them on a stable branch somewhere that you can use.
> 
> Here's what I've done:
> 
> - the two patches are now on a branch[1] based off -rc3 which I
>   officially declare stable. Feel free to rebase your series on top.
> 
> - the KVM fixes branch now embeds this branch (yes, I've rebased it --
>   we'll hopefully survive the outrage).

We don't have a choice to rebase.   I will rebased CS next on [1] and
apply this set on top of it.  Hopefully the KVM fixes will have made it to GKH's
char-misc tree by the time I send him the patches for the next merge window.
Otherwise we'll have to merge patches twice as Catalin mentioned.  We can deal
with that if/when we get there.

Thanks,
Mathieu

> 
> Thanks,
> 
> 	M.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git/log/?h=trace-fixes-5.12
> 
> -- 
> Without deviation from the norm, progress is not possible.