linux-kernel - Re: [RFC 0/8] Provide the EL1 physical timer to the VM

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Date:   Tue, 17 Jan 2017 17:09:12 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Jintack Lim <jintack@...columbia.edu>,
        kvmarm@...ts.cs.columbia.edu, christoffer.dall@...aro.org
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, linux@...linux.org.uk,
        catalin.marinas@....com, will.deacon@....com,
        andre.przywara@....com, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/8] Provide the EL1 physical timer to the VM

On 26/12/16 17:11, 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.
> 
> On VHE systems, this would be as simple as allowing full access to the
> EL1 physical timer to VMs because the KVM host does not use the EL1
> physical timer.  However, on non-VHE systems, the KVM host already uses
> the EL1 physical timer which prevents us from granting full access of
> the EL1 physical timer to VMs.
> 
> This patchset 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 patchset. This patchset will make it easy to add the
> EL2 timer support in the future, though.
> 
> Note, Linux VMs booting in EL1 will be unaffected by this patch set and
> will continue to use only the virtual timer and this patch set will
> therefore not introduce any performance degredation as a result of
> trap-and-emulate.

Hi Jintack,

Any chance you could address Christoffer's comments and respin this
series? This looks like a good enhancement to our emulation, and
definitely a requirement for the nested work, so I'm obviously keen on it.

Thanks,

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