Linux kernel set the hypervisor vector table through ATF

[Mark Rutland]

On Fri, May 28, 2021 at 05:26:51PM +0800, Dongjiu Geng wrote:
> Hi All,
>       when Linux kernel boot from EL1,  there is no method to let
> kernel to enter EL2 to enable hypervisor.  so I want to add an SMC
> interface between kernel and EL3 ATF to let kernel can set the
> hypervisor vector table,  then can enter EL2 to enable hypervisor, as
> shown in [1].
> Do you agree?  Otherwise there is no method to enter EL2 hypervisor
> when kernel boot from EL1, because the hypervisor vector
> table(vbar_el2) is unknown.

The kernel already supported being booted at EL2, where it will install
itself as the hypervisor (and will drop to EL1 if required). EL2 is the
preferred boot mode, as we document in:

https://www.kernel.org/doc/html/latest/arm64/booting.html

... where we say:

| The CPU must be in either EL2 (RECOMMENDED in order to have access to
| the virtualisation extensions) or non-secure EL1.

We *strongly* prefer this over adding new ABIs to transition from EL1 to
EL2. Please boot the kernel at EL2 if you want to use KVM.

Thanks,
Mark.

> 
> 
> 
> [1]:
> el1_sync:
>         cmp     x0, #SMC_SET_VECTORS
>         b.ne    2f
>         msr     vbar_el2, x1
>         b       9f
> 
> 2:      cmp     x0, #SMC_SOFT_RESTART
>         b.ne    3f
>         mov     x0, x2
>         mov     x2, x4
>         mov     x4, x1
>         mov     x1, x3
>         br      x4                              // no return
> 
> 3:      cmp     x0, #SMC_RESET_VECTORS
>         beq     9f                              // Nothing to reset!
> 
>         ldr     x0, =SMC_STUB_ERR
>         eret
> 
> 9:      mov     x0, xzr
>         eret
> ENDPROC(el1_sync)
> 
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