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From: Marc Zyngier <maz@kernel.org>
To: Andrew Murray <andrew.murray@arm.com>
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: Sun, 22 Dec 2019 12:22:10 +0000
Message-ID: <864kxsim8d.wl-maz@kernel.org> (raw)
In-Reply-To: <f023f5529361cc1e2d799daa70f196c2@www.loen.fr>

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?
> 
> 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.

- 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.

	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 [this message]
2019-12-24 12:56     ` Andrew Murray

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