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From: Andrew Murray <andrew.murray@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kvm@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, Sudeep Holla <sudeep.holla@arm.com>,
	will@kernel.org, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 00/18] arm64: KVM: add SPE profiling support
Date: Tue, 24 Dec 2019 12:56:51 +0000
Message-ID: <20191224125651.GM42593@e119886-lin.cambridge.arm.com> (raw)
In-Reply-To: <864kxsim8d.wl-maz@kernel.org>

On Sun, Dec 22, 2019 at 12:22:10PM +0000, Marc Zyngier wrote:
> On Sat, 21 Dec 2019 10:48:16 +0000,
> Marc Zyngier <maz@kernel.org> wrote:
> > 
> > [fixing email addresses]
> > 
> > Hi Andrew,
> > 
> > On 2019-12-20 14:30, Andrew Murray wrote:
> > > This series implements support for allowing KVM guests to use the Arm
> > > Statistical Profiling Extension (SPE).
> > 
> > Thanks for this. In future, please Cc me and Will on email addresses
> > we can actually read.
> > 
> > > It has been tested on a model to ensure that both host and guest can
> > > simultaneously use SPE with valid data. E.g.
> > > 
> > > $ perf record -e arm_spe/ts_enable=1,pa_enable=1,pct_enable=1/ \
> > >         dd if=/dev/zero of=/dev/null count=1000
> > > $ perf report --dump-raw-trace > spe_buf.txt
> > > 
> > > As we save and restore the SPE context, the guest can access the SPE
> > > registers directly, thus in this version of the series we remove the
> > > trapping and emulation.
> > > 
> > > In the previous series of this support, when KVM SPE isn't
> > > supported (e.g. via CONFIG_KVM_ARM_SPE) we were able to return a
> > > value of 0 to all reads of the SPE registers - as we can no longer
> > > do this there isn't a mechanism to prevent the guest from using
> > > SPE - thus I'm keen for feedback on the best way of resolving
> > > this.
> > 
> > Surely there is a way to conditionally trap SPE registers, right? You
> > should still be able to do this if SPE is not configured for a given
> > guest (as we do for other feature such as PtrAuth).
> > 
> > > It appears necessary to pin the entire guest memory in order to
> > > provide guest SPE access - otherwise it is possible for the guest
> > > to receive Stage-2 faults.
> > 
> > Really? How can the guest receive a stage-2 fault? This doesn't fit
> > what I understand of the ARMv8 exception model. Or do you mean a SPE
> > interrupt describing a S2 fault?

Yes the latter.


> > 
> > And this is not just pinning the memory either. You have to ensure that
> > all S2 page tables are created ahead of SPE being able to DMA to guest
> > memory. This may have some impacts on the THP code...
> > 
> > I'll have a look at the actual series ASAP (but that's not very soon).
> 
> I found some time to go through the series, and there is clearly a lot
> of work left to do:
> 
> - There so nothing here to handle memory pinning whatsoever. If it
>   works, it is only thanks to some side effect.
> 
> - The missing trapping is deeply worrying. Given that this is an
>   optional feature, you cannot just let the guest do whatever it wants
>   in an uncontrolled manner.

Yes I'll add this.


> 
> - The interrupt handling is busted. You mix concepts picked from both
>   the PMU and the timer code, while the SPE device doesn't behave like
>   any of these two (it is neither a fully emulated device, nor a
>   device that is exclusively owned by a guest at any given time).
> 
> I expect some level of discussion on the list including at least Will
> and myself before you respin this.

Thanks for the quick feedback.

Andrew Murray

> 
> 	M.
> 
> -- 
> Jazz is not dead, it just smells funny.

      reply index

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-20 14:30 Andrew Murray
2019-12-20 14:30 ` [PATCH v2 01/18] dt-bindings: ARM SPE: highlight the need for PPI partitions on heterogeneous systems Andrew Murray
2019-12-20 14:30 ` [PATCH v2 02/18] arm64: KVM: reset E2PB correctly in MDCR_EL2 when exiting the guest(VHE) Andrew Murray
2019-12-21 13:12   ` Marc Zyngier
2019-12-24 10:29     ` Andrew Murray
2020-01-02 16:21       ` Andrew Murray
2019-12-20 14:30 ` [PATCH v2 03/18] arm64: KVM: define SPE data structure for each vcpu Andrew Murray
2019-12-21 13:19   ` Marc Zyngier
2019-12-24 12:01     ` Andrew Murray
2019-12-20 14:30 ` [PATCH v2 04/18] arm64: KVM: add SPE system registers to sys_reg_descs Andrew Murray
2019-12-20 14:30 ` [PATCH v2 05/18] arm64: KVM/VHE: enable the use PMSCR_EL12 on VHE systems Andrew Murray
2019-12-20 14:30 ` [PATCH v2 06/18] arm64: KVM: split debug save restore across vm/traps activation Andrew Murray
2019-12-20 14:30 ` [PATCH v2 07/18] arm64: KVM/debug: drop pmscr_el1 and use sys_regs[PMSCR_EL1] in kvm_cpu_context Andrew Murray
2019-12-20 14:30 ` [PATCH v2 08/18] arm64: KVM: add support to save/restore SPE profiling buffer controls Andrew Murray
2019-12-21 13:57   ` Marc Zyngier
2019-12-24 10:49     ` Andrew Murray
2019-12-24 15:17       ` Andrew Murray
2019-12-24 15:48         ` Marc Zyngier
2019-12-20 14:30 ` [PATCH v2 09/18] arm64: KVM: enable conditional save/restore full " Andrew Murray
2019-12-20 18:06   ` Mark Rutland
2019-12-24 12:15     ` Andrew Murray
2019-12-21 14:13   ` Marc Zyngier
2020-01-07 15:13     ` Andrew Murray
2020-01-08 11:17       ` Marc Zyngier
2020-01-08 11:58         ` Will Deacon
2020-01-08 12:36           ` Marc Zyngier
2020-01-08 13:10             ` Will Deacon
2020-01-09 11:23               ` Andrew Murray
2020-01-09 11:25                 ` Andrew Murray
2020-01-09 12:01                   ` Will Deacon
2020-01-10 10:54     ` Andrew Murray
2020-01-10 11:04       ` Andrew Murray
2020-01-10 11:51         ` Marc Zyngier
2020-01-10 12:12           ` Andrew Murray
2020-01-10 11:18       ` Marc Zyngier
2020-01-10 12:12         ` Andrew Murray
2020-01-10 13:34           ` Marc Zyngier
2019-12-20 14:30 ` [PATCH v2 10/18] arm64: KVM/debug: use EL1&0 stage 1 translation regime Andrew Murray
2019-12-22 10:34   ` Marc Zyngier
2019-12-24 11:11     ` Andrew Murray
2020-01-13 16:31     ` Andrew Murray
2020-01-15 14:03       ` Marc Zyngier
2019-12-20 14:30 ` [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling controls to EL2 Andrew Murray
2019-12-20 18:08   ` Mark Rutland
2019-12-22 10:42   ` Marc Zyngier
2019-12-23 11:56     ` Andrew Murray
2019-12-23 12:05       ` Marc Zyngier
2019-12-23 12:10         ` Andrew Murray
2020-01-09 17:25           ` Andrew Murray
2020-01-09 17:42             ` Mark Rutland
2020-01-09 17:46               ` Andrew Murray
2019-12-20 14:30 ` [PATCH v2 12/18] KVM: arm64: add a new vcpu device control group for SPEv1 Andrew Murray
2019-12-22 11:03   ` Marc Zyngier
2019-12-24 12:30     ` Andrew Murray
2019-12-20 14:30 ` [PATCH v2 13/18] perf: arm_spe: Add KVM structure for obtaining IRQ info Andrew Murray
2019-12-22 11:24   ` Marc Zyngier
2019-12-24 12:35     ` Andrew Murray
2019-12-20 14:30 ` [PATCH v2 14/18] KVM: arm64: spe: Provide guest virtual interrupts for SPE Andrew Murray
2019-12-22 12:07   ` Marc Zyngier
2019-12-24 11:50     ` Andrew Murray
2019-12-24 12:42       ` Marc Zyngier
2019-12-24 13:08         ` Andrew Murray
2019-12-24 13:22           ` Marc Zyngier
2019-12-24 13:36             ` Andrew Murray
2019-12-24 13:46               ` Marc Zyngier
2019-12-20 14:30 ` [PATCH v2 15/18] perf: arm_spe: Handle guest/host exclusion flags Andrew Murray
2019-12-20 18:10   ` Mark Rutland
2019-12-22 12:10   ` Marc Zyngier
2019-12-23 12:10     ` Andrew Murray
2019-12-23 12:18       ` Marc Zyngier
2019-12-20 14:30 ` [PATCH v2 16/18] KVM: arm64: enable SPE support Andrew Murray
2019-12-20 14:30 ` [PATCH v2 17/18, KVMTOOL] update_headers: Sync kvm UAPI headers with linux v5.5-rc2 Andrew Murray
2019-12-20 14:30 ` [PATCH v2 18/18, KVMTOOL] kvm: add a vcpu feature for SPEv1 support Andrew Murray
2019-12-20 17:55 ` [PATCH v2 00/18] arm64: KVM: add SPE profiling support Mark Rutland
2019-12-24 12:54   ` Andrew Murray
2019-12-21 10:48 ` Marc Zyngier
2019-12-22 12:22   ` Marc Zyngier
2019-12-24 12:56     ` Andrew Murray [this message]

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