Re: [Minios-devel] [UNIKRAFT PATCHv4 21/43] plat/kvm: Add Arm64 basic entry code

[Julien Grall <julien.grall@xxxxxxx>]

On 11/07/18 10:50, Wei Chen wrote:
> Hi Julien,
Hi Wei,

> > -----Original Message-----
> > From: Julien Grall <julien.grall@xxxxxxx>
> > Sent: 2018年7月8日 6:24
> > To: Wei Chen <Wei.Chen@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> > simon.kuenzer@xxxxxxxxx
> > Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> > Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 21/43] plat/kvm: Add Arm64 basic
> > entry code
> >
> > Hi,
> >
> > On 07/06/2018 10:03 AM, Wei Chen wrote:
> > > QEMU/KVM can boot an Arm64 elf image without multiboot. In this case,
> > > we can plage _libkvmplat_entry to entry64.S directly as the vCPU
> > > reset entry. In this basic entry code, we just initialize the boot
> > > stack and prepare jumping to _libkvmplat_start.
> > Can you clarify why you are using the ELF format and not Image? My main
> > concern is the former does not seem to have a clear description of the
> > state of the VM at boot.
> It's little hard for me to answer your question. This is why I reply this
> Comment at the last. Actually, when I was selecting the elf image I didn’t
> think so much. And most Unikernel projects that I have involved (ukvm, mini-os)
> are using the elf image, both for arm and x86.
Mini-OS ARM is using the zImage format, not ELF. For UKVM, IIRC, you 
wrote it yourself. So I guess you based your understanding of the state 
of the VM from somewhere?
> So I don't know and understand your concern. Could please give me a
> detail of "clear description of the state of the VM at boot" ?
My concern is you don't know what is the state of the memory, caches, 
CPU mode... For the Image protocol, this is clearly identified in [1].
If you tell me ELF has the same description somewhere, then fine. I will 
be happy to look at it and compare with what you did.
> > For instance, it is not clear what is the state of the cache, SCTLR...
> If we use other format image can we get above information? How does it do this?
Yes, this is described in the format documentation [1].

> > You also assume the MMU is turned on. Do you have a pointer on what is
> > the expected state at boot? This would be quite useful to review the
> > boot code.
> I don't have the pointer, I just refer to FreeBSD's steps.
AFAIK, FreeBSD has its own bootloader based on UEFI. This is different 
from booting without firmware.
>   
> > > Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
> > > ---
> > >    plat/kvm/arm/entry64.S | 36 ++++++++++++++++++++++++++++++
> > >    plat/kvm/arm/setup.c   | 50 ++++++++++++++++++++++++++++++++++++++++++
> > >    2 files changed, 86 insertions(+)
> > >    create mode 100644 plat/kvm/arm/entry64.S
> > >    create mode 100644 plat/kvm/arm/setup.c
> > >
> > > diff --git a/plat/kvm/arm/entry64.S b/plat/kvm/arm/entry64.S
> > > new file mode 100644
> > > index 0000000..8a8a2e0
> > > --- /dev/null
> > > +++ b/plat/kvm/arm/entry64.S
> > > @@ -0,0 +1,36 @@
> > > +#include <uk/arch/limits.h>
> > > +#include <arm/cpu_defs.h>
> > > +
> > > +.data
> > > +.globl _dtb
> > > +
> > > +#define BOOT_STACK_SIZE PAGE_SIZE
> > > +
> > > +/*
> > > + * The registers used by _libkvmplat_start:
> > > + * x0 - FDT pointer
> > > + */
> > > +
> > > +.text
> > > +ENTRY(_libkvmplat_entry)
> > > +       /* Boot stack is placed after pagetable area temporarily */
> > > +       ldr x26, =_end
> > > +       add x26, x26, #PAGE_TABLE_SIZE
> > > +       add x27, x26, #BOOT_STACK_SIZE
> > > +
> > > +       /* Clean the boot stack */
> > > +1:
> > > +       stp xzr, xzr, [x26], #16
> > > +       stp xzr, xzr, [x26], #16
> > > +       stp xzr, xzr, [x26], #16
> > > +       stp xzr, xzr, [x26], #16
> > I guess you expect the stack to be 64-byte aligned? If so, It would be
> > nice to write it down in a comment.
> Why did you have such feeling? I think my stack is 16-bytes alignment.
Do you agree that each stp instruction will clear 16-byte? So if you 
execute 4 stp consecutively without boundary check, you impose the stack 
to be 64-byte aligned.
Did I miss anything?

Cheers,

[1] https://www.kernel.org/doc/Documentation/arm64/booting.txt

--
Julien Grall

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