linux-kernel - Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM

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Date:   Fri, 3 Feb 2017 14:34:37 +0100
From:   Christoffer Dall <cdall@...aro.org>
To:     Jintack Lim <jintack@...columbia.edu>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andre Przywara <andre.przywara@....com>,
        KVM General <kvm@...r.kernel.org>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM

On Fri, Feb 3, 2017 at 2:14 PM, Jintack Lim <jintack@...columbia.edu> wrote:
> On Fri, Feb 3, 2017 at 7:33 AM, Christoffer Dall <cdall@...aro.org> wrote:
>> On Thu, Feb 02, 2017 at 09:51:13AM -0500, Jintack Lim wrote:
>>> On Thu, Feb 2, 2017 at 7:31 AM, Christoffer Dall <cdall@...aro.org> wrote:
>>> > Hi Jintack,
>>> >
>>> > On Wed, Feb 01, 2017 at 12:43:00PM -0500, Jintack Lim wrote:
>>> >> The ARM architecture defines the EL1 physical timer and the virtual timer,
>>> >> and it is reasonable for an OS to expect to be able to access both.
>>> >> However, the current KVM implementation does not provide the EL1 physical
>>> >> timer to VMs but terminates VMs on access to the timer.
>>> >>
>>> >> This patch series enables VMs to use the EL1 physical timer through
>>> >> trap-and-emulate.  The KVM host emulates each EL1 physical timer register
>>> >> access and sets up the background timer accordingly.  When the background
>>> >> timer expires, the KVM host injects EL1 physical timer interrupts to the
>>> >> VM.  Alternatively, it's also possible to allow VMs to access the EL1
>>> >> physical timer without trapping.  However, this requires somehow using the
>>> >> EL2 physical timer for the Linux host while running the VM instead of the
>>> >> EL1 physical timer.  Right now I just implemented trap-and-emulate because
>>> >> this was straightforward to do, and I leave it to future work to determine
>>> >> if transferring the EL1 physical timer state to the EL2 timer provides any
>>> >> performance benefit.
>>> >>
>>> >> This feature will be useful for any OS that wishes to access the EL1
>>> >> physical timer. Nested virtualization is one of those use cases. A nested
>>> >> hypervisor running inside a VM would think it has full access to the
>>> >> hardware and naturally tries to use the EL1 physical timer as Linux would
>>> >> do. Other nested hypervisors may try to use the EL2 physical timer as Xen
>>> >> would do, but supporting the EL2 physical timer to the VM is out of scope
>>> >> of this patch series. This patch series will make it easy to add the EL2
>>> >> timer support in the future, though.
>>> >>
>>> >> Note that Linux VMs booting in EL1 will be unaffected by this patch series
>>> >> and will continue to use only the virtual timer and this patch series will
>>> >> therefore not introduce any performance degredation as a result of
>>> >> trap-and-emulate.
>>> >>
>>> >> v2 => v3:
>>> >>  - Rebase on kvmarm/queue
>>> >>  - Take kvm->lock to synchronize cntvoff across all vtimers
>>> >>  - Remove unnecessary function parameters
>>> >>  - Add comments
>>> >
>>> > I just gave v3 a test run on my TC2 (32-bit platform) and my guest
>>> > quickly locks up trying to run cyclictest or when booting the machine it
>>> > stalls with RCU timeouts.
>>>
>>> Ok. It's my fault not to specify that the emulated physical timer is
>>> supported/tested on arm64.
>>> On 32-bit platform, it is supposed to show the same behavior as
>>> before, but I haven't tested.
>>> Were you using the physical timer or the virtual timer for the guest?
>>>
>>> >
>>> > Could you have a look?
>>>
>>> Sure, I'll have a look. I don't have access to my Cubietruck today,
>>> but I can work on that tomorrow.
>>>
>>
>> Don't bother, I've figured this out for you.
>
> Thanks a lot.
>
>>
>> You need the following fixup to your patch:
>
> Ok. I'll post v4 soon.
> You've already do "acked-by" for this commit. Do I need to change it
> to "signed-off-by"?
>

I guess so, technically.  I don't care deeply though.

-Christoffer