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Re: [Xen-devel] struct vcpu_guest_core_reg stable ABI?
- To: Julien Grall <julien.grall@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, "Stefano Stabellini" <sstabellini@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 15 May 2019 21:12:37 +0100
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- Delivery-date: Wed, 15 May 2019 20:12:54 +0000
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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.
No point lumbering yourself with a broke structure in the API if you can
avoid it.
~Andrew
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