Re: [PATCH v4 28/28] arm/arm64: Add hyp-stub API documentation

Christoffer Dall
2017-03-24T14:33:16Z

On Tue, Mar 21, 2017 at 07:20:58PM +0000, Marc Zyngier wrote:
> In order to help people understanding the hyp-stub API that exists
> between the host kernel and the hypervisor mode (whether a hypervisor
> has been installed or not), let's document said API.
>
> As with any form of documentation, I expect it to become obsolete
> and completely misleading within 20 minutes after having being merged.

I don't think this last sentence belongs in the commit message.

>
> Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> Documentation/virtual/kvm/arm/hyp-abi.txt | 45 +++++++++++++++++++++++++++++++
> 1 file changed, 45 insertions(+)
> create mode 100644 Documentation/virtual/kvm/arm/hyp-abi.txt
>
> diff --git a/Documentation/virtual/kvm/arm/hyp-abi.txt b/Documentation/virtual/kvm/arm/hyp-abi.txt
> new file mode 100644
> index 000000000000..a1e0314d2249
> --- /dev/null
> +++ b/Documentation/virtual/kvm/arm/hyp-abi.txt
> @@ -0,0 +1,45 @@
> +* Internal ABI between the kernel and HYP
> +
> +This file documents the interaction between the Linux kernel and the
> +hypervisor layer when running Linux as a hypervisor (for example
> +KVM). It doesn't cover the interaction of the kernel with the
> +hypervisor when running as a guest (under Xen, KVM or any other
> +hypervisor), or any hypervisor-specific interaction when the kernel is
> +used as a host.
> +
> +On arm and arm64 (without VHE), the kernel doesn't run in hypervisor
> +mode, but still needs to interact with it, allowing a built-in
> +hypervisor to be either installed or torn down.
> +
> +In order to achieve this, the kernel must be booted at HYP (arm) or
> +EL2 (arm64), allowing it to install a set of stubs before dropping to
> +SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction,
> +and only act on individual CPUs.
> +
> +Unless specified otherwise, any built-in hypervisor must implement
> +these functions (see arch/arm{,64}/include/asm/virt.h):
> +
> +* r0/x0 = HVC_SET_VECTORS
> + r1/x1 = vectors
> +
> + Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors'
> + must be a physical address, and respect the alignment requirements
> + of the architecture. Only implemented by the initial stubs.

Does this last sentence mean that KVM doesn't implement this function?

> +
> +* r0/x0 = HVC_RESET_VECTORS
> +
> + Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the default
> + value. This effectively disables an existing hypervisor.

What's the 'default value' ? Could we say to the physical address of
the hypervisor stub's exception vector?

> +
> +* r0/x0 = HVC_SOFT_RESTART
> + r1/x1 = restart address
> + x2 = x0's value when entering the next payload (arm64)
> + x3 = x1's value when entering the next payload (arm64)
> + x4 = x2's value when entering the next payload (arm64)
> +
> + Mask all exceptions, disable the MMU, move the arguments into place
> + (arm64 only), and jump to the restart address while at HYP/EL2. This
> + hypercall is not expected to return to its caller.
> +
> +Any other value of r0/x0 triggers a hypervisor-specific handling,
> +which is not documented here.
> --
> 2.11.0
>

Thanks,
-Christoffer