Re: [PATCH] KVM: arm64: Prevent vcpu_has_ptrauth from generating OOL functions

[Marc Zyngier <maz@kernel.org>]

Hi Nathan,

On 2020-07-23 03:51, Nathan Chancellor wrote:
> On Wed, Jul 22, 2020 at 05:22:31PM +0100, Marc Zyngier wrote:
>> So far, vcpu_has_ptrauth() is implemented in terms of 
>> system_supports_*_auth()
>> calls, which are declared "inline". In some specific conditions (clang
>> and SCS), the "inline" very much turns into an "out of line", which
>> leads to a fireworks when this predicate is evaluated on a non-VHE
>> system (right at the beginning of __hyp_handle_ptrauth).
>> 
>> Instead, make sure vcpu_has_ptrauth gets expanded inline by directly
>> using the cpus_have_final_cap() helpers, which are __always_inline,
>> generate much better code, and are the only thing that make sense when
>> running at EL2 on a nVHE system.
>> 
>> Fixes: 29eb5a3c57f7 ("KVM: arm64: Handle PtrAuth traps early")
>> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
>> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
> 
> Thank you for the quick fix! I have booted a mainline kernel with this
> patch with Shadow Call Stack enabled and verified that using KVM no
> longer causes a panic.

Great! I'll try and ferry this to mainline  as quickly as possible.

> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> 
> For the future, is there an easy way to tell which type of system I am
> using (nVHE or VHE)? I am new to the arm64 KVM world but it is 
> something
> that I am going to continue to test with various clang technologies now
> that I have actual hardware capable of it that can run a mainline
> kernel.

ARMv8.0 CPUs are only capable of running non-VHE. So if you have
something based on older ARM CPUs (such as A57, A72, A53, A73, A35...),
or licensee CPUs (ThunderX, XGene, EMag...), this will only run
non-VHE (the host kernel runs at EL1, while the hypervisor runs at
EL2.

 From ARMv8.1 onward, VHE is normally present, and the host kernel
can run at EL2 directly. ARM CPUs include A55, A65, A75, A76, A77,
N1, while licensee CPUs include TX2, Kunpeng 920, and probably some
more.

As pointed out by Zenghui in another email, KVM shows which mode
it is using. Even without KVM, the kernel prints very early on:

[    0.000000] CPU features: detected: Virtualization Host Extensions

Note that this is only a performance difference, and that most
features that are supported by the CPU can be used by KVM in either
mode.

Thanks again,

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