Re: [PATCH 2/2] ARM: OMAP5: Add HYP mode entry support for secondary CPUs

[Christoffer Dall <christoffer.dall@linaro.org>]

On 25 November 2013 08:28, Santosh Shilimkar <santosh.shilimkar@ti.com> wrote:
> On Monday 25 November 2013 10:09 AM, Christoffer Dall wrote:
>> On 23 November 2013 16:07, Santosh Shilimkar <santosh.shilimkar@ti.com> wrote:
>>> Boot-CPU entry into the HYP mode is managed in boot-loader but
>>> the secondary CPUs directly jumps to kernel during boot. Same
>>> path is also used for CPU hotplug as well during suspend for
>>> secondary CPU.
>>>
>>> Hence patch the secondary CPU boot path for hyp mode etry.
>>>
>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>>> Cc: Tony Lindgren <tony@atomide.com>
>>> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
>>> ---
>>>  arch/arm/mach-omap2/omap-headsmp.S |    7 +++++++
>>>  1 file changed, 7 insertions(+)
>>>
>>> diff --git a/arch/arm/mach-omap2/omap-headsmp.S b/arch/arm/mach-omap2/omap-headsmp.S
>>> index 75e9295..4844dd8 100644
>>> --- a/arch/arm/mach-omap2/omap-headsmp.S
>>> +++ b/arch/arm/mach-omap2/omap-headsmp.S
>>> @@ -22,6 +22,7 @@
>>>
>>>  /* Physical address needed since MMU not enabled yet on secondary core */
>>>  #define AUX_CORE_BOOT0_PA                      0x48281800
>>> +#define API_HYP_ENTRY                          0x102
>>>
>>>  /*
>>>   * OMAP5 specific entry point for secondary CPU to jump from ROM
>>> @@ -38,6 +39,12 @@ wait:        ldr     r2, =AUX_CORE_BOOT0_PA  @ read from AuxCoreBoot0
>>>         and     r4, r4, #0x0f
>>>         cmp     r0, r4
>>>         bne     wait
>>> +#ifdef CONFIG_KVM_ARM_HOST
>>> +       ldr     r12, =API_HYP_ENTRY
>>> +       adr     r0, hyp_boot
>>> +       smc     #0
>>> +hyp_boot:
>>> +#endif
>>>         b       secondary_startup
>>>  END(omap5_secondary_startup)
>>>  /*
>>
>> hmm, this means that currently running this in a guest will fail to
>> bring-up SMP, right?
>>
> Nope. Because the code under 'KVM_ARM_HOST' macro. Guest build
> will not enable CONFIG_KVM_ARM_HOST and things should be fine then.
> Right ?
>

That really goes against the whole single binary on all platforms
thing. With multi-platform support you really shouldn't have to
compile your kernel any differently for running as a guest as when
you're running on a host.  Someone may even emulate an OMAP5 in QEMU
and you'd certainly want your kvm-enabled kernel to run as both guest
and host.  After all, this is not a paravirtualization solution.

>> Couldn't you create a little wrapper-pen in U-Boot instead, which
>> replicates the omap boot protocol and takes care of the hyp-mode
>> startup there instead, keeping this completely out of the kernel?
>>
> Its not just booting but CPU hotplug also follows the same path
> so we need the mechanism in kernel to switch mode.
>
> In general, I think its important to consider the aspect with
> CPU PM. CPUs are not going to go through the boot-loaders in
> those paths and hence need of HYP entry in the kernel will
> be must.
>
I agree, and PSCI is the obvious only correct answer to this.

We have discussed this a bit earlier (I think Will Deacon brought this
up - cc'ed), but I don't think anyone had any bright ideas.

However, we broadly agreed on the fact that for KVM/hyp support, you
need to boot your kernel in that mode, and this is definitely pulling
in the wrong direction.

-Christoffer