Re: [Xen-devel] struct vcpu_guest_core_reg stable ABI?

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrew & Jan,

On 5/16/19 8:58 AM, Jan Beulich wrote:
> > > > On 15.05.19 at 22:12, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 15/05/2019 20:58, Julien Grall wrote:
> > > Hi all,
> > >
> > > It looks like the structures vcpu_guest_core_regs and
> > > vcpu_guest_context does not correctly reflect the AArch64 state. For
> > > instance, all Arm64 system registers (e.g sctlr, cpsr, spsr_el1)
> > > should be 64-bit wide not 32-bit wide.
> > >
> > > On ARMv8.0, some of the registers have only the low 32-bit defined,
> > > the rest is RES0. RES0 only means they are reserved for future use, it
> > > does not mean they can be ignored. Newer revision (such as
> > > ARMv8.0-SSBS) actually began to define bit in the top 32-bit.
> > >
> > > This means that the structures vcpu_guest_core_regs and
> > > vcpu_guest_context would not be able to store the top 32-bit and
> > > therefore misrepresenting the hardware.
> > >
> > >  From my understanding, vcpu_guest_context is defined between the tools
> > > and Xen. So it would be possible to modify it without caring on
> > > backward compatibly.
> > >
> > > Howerver, struct vcpu_guest_core_reg seems to be outside of the
> > > #ifdef. So I assume it is part of the stable ABI. Am I correct?
> > >
> > > Do you have any suggestion how this could safely be extended?
> > Stuff like this should never have been in the public API to begin with.
> > x86 has some nasty issues which I have yet to find a good-enough way to fix.
> >
> > For ARM, and future architectures, I'd use the fact that there are no
> > non-tools interfaces which use this structure to allow yourself the
> > wiggleroom to declare history a mistake, and fix it by making it tools-only.
> I'm unconvinced of the "declare history a mistake" part, but I agree
> with the suggestion of simply moving the structure declaration down
> into the guarded area. It simply was a mistake to not put it there in
> the first place.
Thank you both of you for the feedback. I will prepare a patch to make 
struct vcpu_guest_core_regs tools only.
Cheers,

--
Julien Grall

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