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 17:35:06 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Marc Zyngier <maz@...nel.org>,
        Catalin Marinas <catalin.marinas@....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@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 07/19] arm64: kvm: Enable access to TRBE support for
 host

On Tue, Mar 30, 2021 at 09:23:14AM -0600, Mathieu Poirier 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.

My tree can not be based on -next, and neither can any other
maintainer's tree, as next is composed of maintainer trees :)

> > 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.  The
> usual way to work with complex merge dependencies is to proceed in steps, which
> would mean that all KVM related patches go in the v5.13 merge window.  When that
> is done we add the ETE/TRBE for the v5.14 merge window.  I agree that we waste
> an entire cycle but it guarantees to avoid breaking builds and follows the
> conventional way to do things.   

Or someone creates a single branch with a signed tag and it gets pulled
into multiple maintainer's trees and never rebased.  We've done that
lots of time, nothing new there.  Or everything goes through one tree,
or you wait a release cycle.

You have 3 choices, pick one :)

thanks,

greg k-h