Re: [PATCH] arm(64)/kvm: improve the documentation about HVC calls

[Pingfan Liu <kernelfans@gmail.com>]

On Thu, Aug 27, 2020 at 2:10 AM James Morse <james.morse@arm.com> wrote:
>
> Hi Pingfan,
>
> On 12/08/2020 15:05, Pingfan Liu wrote:
> > Both arm and arm64 kernel entry point have the following prerequisite:
> >   MMU = off, D-cache = off, I-cache = dont care.
> >
> > HVC_SOFT_RESTART call should meet this prerequisite before jumping to the
> > new kernel.
>
> I think you have this the wrong way up. This should describe what HVC_SOFT_RESTART does.
Yes, it is a wrong way.

>
> We want to remove some extra work kexec does on arm64, and both implementations of
> HVC_SOFT_RESTART on arm64 already do what we need. The change here should be to document
> that the D/I bits are cleared after a HVC_SOFT_RESTART on arm64.
>
>
> > Furthermore, on arm64, el2_setup doesn't set I+C bits and keeps EL2 MMU
> > off, and KVM resets them when its unload. These are achieved by
> > HVC_RESET_VECTORS call.
> >
> > Improve the document.
>
>
> > diff --git a/Documentation/virt/kvm/arm/hyp-abi.rst b/Documentation/virt/kvm/arm/hyp-abi.rst
> > index d9eba93..a95bc30 100644
> > --- a/Documentation/virt/kvm/arm/hyp-abi.rst
> > +++ b/Documentation/virt/kvm/arm/hyp-abi.rst
> > @@ -40,9 +40,9 @@ these functions (see arch/arm{,64}/include/asm/virt.h):
> >
> >  * ::
> >
> > -    r0/x0 = HVC_RESET_VECTORS
> > +    x0 = HVC_RESET_VECTORS (arm64 only)
> >
> > -  Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials
> > +  Disable HYP/EL2 MMU and D-cache, and reset HVBAR/VBAR_EL2 to the initials
> >    stubs' exception vector value. This effectively disables an existing
> >    hypervisor.
>
> I don't think we should remove this. KVM on 32bit was the only implementer, but if there
> ever is another, this is how it should work.
>
>
> > @@ -54,7 +54,7 @@ these functions (see arch/arm{,64}/include/asm/virt.h):
> >      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
> > +  Mask all exceptions, disable the MMU and D-cache, 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.
>
> (I don't think disable the D-cache is what the bit does, it forces the attributes that are
> used for a data access).
>
> Please just describe this as the on arm64 the D and I bits are cleared.
OK, I will do it.

> (it might be true on 32bit, I can't work the assembly out).
I will leave 32bit as it is.

Thanks,
Pingfan