linux-kernel - Re: [PATCH v5 07/19] arm64: kvm: Enable access to TRBE support for host

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Date:   Tue, 30 Mar 2021 16:34:45 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        gregkh@...uxfoundation.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@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 07/19] arm64: kvm: Enable access to TRBE support for host

On Tue, 30 Mar 2021 16:23:14 +0100,
Mathieu Poirier <mathieu.poirier@...aro.org> wrote:
> 
> On Tue, Mar 30, 2021 at 11:38:18AM +0100, Suzuki K Poulose wrote:
> > On 26/03/2021 16:55, Mathieu Poirier wrote:
> > > On Tue, Mar 23, 2021 at 12:06:35PM +0000, Suzuki K Poulose wrote:
> > > > For a nvhe host, the EL2 must allow the EL1&0 translation
> > > > regime for TraceBuffer (MDCR_EL2.E2TB == 0b11). This must
> > > > be saved/restored over a trip to the guest. Also, before
> > > > entering the guest, we must flush any trace data if the
> > > > TRBE was enabled. And we must prohibit the generation
> > > > of trace while we are in EL1 by clearing the TRFCR_EL1.
> > > > 
> > > > For vhe, the EL2 must prevent the EL1 access to the Trace
> > > > Buffer.
> > > > 
> > > > Cc: Will Deacon <will@...nel.org>
> > > > Cc: Catalin Marinas <catalin.marinas@....com>
> > > > Cc: Marc Zyngier <maz@...nel.org>
> > > > Cc: Mark Rutland <mark.rutland@....com>
> > > > Cc: Anshuman Khandual <anshuman.khandual@....com>
> > > > Acked-by: Mathieu Poirier <mathieu.poirier@...aro.org>
> > > > Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> > > > ---
> > > >   arch/arm64/include/asm/el2_setup.h | 13 +++++++++
> > > >   arch/arm64/include/asm/kvm_arm.h   |  2 ++
> > > >   arch/arm64/include/asm/kvm_host.h  |  2 ++
> > > >   arch/arm64/kernel/hyp-stub.S       |  3 ++-
> > > >   arch/arm64/kvm/debug.c             |  6 ++---
> > > >   arch/arm64/kvm/hyp/nvhe/debug-sr.c | 42 ++++++++++++++++++++++++++++++
> > > >   arch/arm64/kvm/hyp/nvhe/switch.c   |  1 +
> > > >   7 files changed, 65 insertions(+), 4 deletions(-)
> > > > 
> > > 
> > > Marc - do you want me to pick up this one?
> > 
> > I think the kvmarm tree is the best route for this patch, given the amount
> > of changes the tree is going through, in the areas this patch
> > touches. Or else there would be conflicts with merging. And this patch
> > depends on the patches from this series that were queued.
> > 
> > Here is the depency tree :
> > 
> > a) kvm-arm fixes for debug (Patch 1, 2) & SPE save-restore fix (queued in
> > v5.12-rc3)
> > 
> > b) TRBE defintions and Trace synchronization barrier (Patches 5 & 6)
> > 
> > c) kvm-arm TRBE host support (Patch 7)
> > 
> > d) TRBE driver support (and the ETE changes)
> > 
> > 
> > (c) code merge depends on -> (a) + (b)
> > (d) build (no conflicts) depends on -> (b)
> > 
> > 
> > Now (d) has an indirect dependency on (c) for operational correctness at
> > runtime.
> > So, if :
> > 
> > kvmarm tree picks up : b + c
> > coresight tree picksup : b + d
> > 
> > and if we could ensure the merge order of the trees are in
> > kvmarm
> > greg-kh (device-misc tree) (coresight goes via this tree)
> >
> 
> Greg's char-misc tree is based on the rc releases rather than next.  As such it
> is a while before other branches like kvmarm get merged, causing all sort of
> compilation breakage.
>  
> > we should be fine.
> > 
> > Additionally, we could rip out the Kconfig changes from the TRBE patch
> > and add it only at the rc1, once we verify both the trees are in to make
> > sure the runtime operation dependency is not triggered.
> >
> 
> We could also do that but Greg might frown at the tactic, and
> rightly so.

We do that all the times. Otherwise, it is hardly possible to build an
infrastructure that spans across multiple subsystems *and* involves
userspace. I really wouldn't worry about that.

	M.

-- 
Without deviation from the norm, progress is not possible.