linux-kernel - Re: [PATCH v2 00/21] arm64: Virtualization Host Extension support

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Message-ID: <56A674A7.9010208@arm.com>
Date:	Mon, 25 Jan 2016 19:16:55 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 00/21] arm64: Virtualization Host Extension support

On 25/01/16 16:44, Will Deacon wrote:
> On Mon, Jan 25, 2016 at 04:37:39PM +0000, Marc Zyngier wrote:
>> On 25/01/16 16:26, Will Deacon wrote:
>>> On Mon, Jan 25, 2016 at 03:53:34PM +0000, Marc Zyngier wrote:
>>>> ARMv8.1 comes with the "Virtualization Host Extension" (VHE for
>>>> short), which enables simpler support of Type-2 hypervisors.
>>>>
>>>> This extension allows the kernel to directly run at EL2, and
>>>> significantly reduces the number of system registers shared between
>>>> host and guest, reducing the overhead of virtualization.
>>>>
>>>> In order to have the same kernel binary running on all versions of the
>>>> architecture, this series makes heavy use of runtime code patching.
>>>>
>>>> The first 20 patches massage the KVM code to deal with VHE and enable
>>>> Linux to run at EL2. The last patch catches an ugly case when VHE
>>>> capable CPUs are paired with some of their less capable siblings. This
>>>> should never happen, but hey...
>>>>
>>>> I have deliberately left out some of the more "advanced"
>>>> optimizations, as they are likely to distract the reviewer from the
>>>> core infrastructure, which is what I care about at the moment.
>>>>
>>>> A few things to note:
>>>>
>>>> - Given that the code has been almost entierely rewritten, I've
>>>>   dropped all Acks from the new patches
>>>>
>>>> - GDB is currently busted on VHE systems, as it checks for version 6
>>>>   on the debug architecture, while VHE is version 7. The binutils
>>>>   people are on the case.
>>>
>>> [...]
>>>
>>>>  arch/arm/include/asm/virt.h          |   5 ++
>>>>  arch/arm/kvm/arm.c                   | 151 +++++++++++++++++++------------
>>>>  arch/arm/kvm/mmu.c                   |   7 ++
>>>>  arch/arm64/Kconfig                   |  13 +++
>>>>  arch/arm64/include/asm/cpufeature.h  |   3 +-
>>>>  arch/arm64/include/asm/kvm_arm.h     |   1 +
>>>>  arch/arm64/include/asm/kvm_emulate.h |   3 +
>>>>  arch/arm64/include/asm/kvm_mmu.h     |  34 ++++++-
>>>>  arch/arm64/include/asm/virt.h        |  27 ++++++
>>>>  arch/arm64/kernel/asm-offsets.c      |   3 -
>>>>  arch/arm64/kernel/cpufeature.c       |  15 +++-
>>>>  arch/arm64/kernel/head.S             |  51 ++++++++++-
>>>>  arch/arm64/kernel/smp.c              |   3 +
>>>>  arch/arm64/kvm/hyp-init.S            |  18 +---
>>>>  arch/arm64/kvm/hyp.S                 |   7 ++
>>>>  arch/arm64/kvm/hyp/entry.S           |   6 ++
>>>>  arch/arm64/kvm/hyp/hyp-entry.S       | 107 +++++++---------------
>>>>  arch/arm64/kvm/hyp/hyp.h             | 119 ++++++++++++++++++++++--
>>>>  arch/arm64/kvm/hyp/switch.c          | 170 +++++++++++++++++++++++++++++++----
>>>>  arch/arm64/kvm/hyp/sysreg-sr.c       | 147 ++++++++++++++++++++----------
>>>>  arch/arm64/kvm/hyp/timer-sr.c        |  10 +--
>>>>  drivers/clocksource/arm_arch_timer.c |  96 ++++++++++++--------
>>>>  22 files changed, 724 insertions(+), 272 deletions(-)
>>>
>>> Have you tried hw_breakpoint/perf/ptrace with these changes? I was under
>>> the impression that the debug architecture was aware of E2H and did need
>>> some changes made. I know you say that GDB is broken anyway, but we should
>>> check that the kernel does the right thing if userspace pokes it the
>>> right way.
>>
>> I did use HW breakpoints on the model by hacking the host kernel to
>> return Debug Version 6 instead of 7, and things seem to work as
>> expected. strace also works out of the box.
>>
>> As for perf, did you have something precise in mind?
> 
> It would be worth trying things like the filter options on perf events
> (perf stat -e cycles:k to count cycles in kernel space) and also
> breakpoints (perf stat -e mem:<addr>:rwx on kernel addresses).

So indeed these didn't work (perf reported 0 for kernel accesses). The
fixes are pretty trivial, and I've put them on top of my kvm-arm64/vhe
branch, for those who want to have a look.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...