Re: [PATCH v2 07/14] xen/riscv: introduce exception context

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 30/01/2023 13:50, Jan Beulich wrote:
> On 30.01.2023 12:54, Oleksii wrote:
> > Hi Jan,
> >
> > On Fri, 2023-01-27 at 15:24 +0100, Jan Beulich wrote:
> > > On 27.01.2023 14:59, Oleksii Kurochko wrote:
> > > > --- /dev/null
> > > > +++ b/xen/arch/riscv/include/asm/processor.h
> > > > @@ -0,0 +1,82 @@
> > > > +/* SPDX-License-Identifier: MIT */
> > > > +/*****************************************************************
> > > > *************
> > > > + *
> > > > + * Copyright 2019 (C) Alistair Francis <alistair.francis@xxxxxxx>
> > > > + * Copyright 2021 (C) Bobby Eshleman <bobby.eshleman@xxxxxxxxx>
> > > > + * Copyright 2023 (C) Vates
> > > > + *
> > > > + */
> > > > +
> > > > +#ifndef _ASM_RISCV_PROCESSOR_H
> > > > +#define _ASM_RISCV_PROCESSOR_H
> > > > +
> > > > +#ifndef __ASSEMBLY__
> > > > +
> > > > +/* On stack VCPU state */
> > > > +struct cpu_user_regs
> > > > +{
> > > > +    unsigned long zero;
> > > > +    unsigned long ra;
> > > > +    unsigned long sp;
> > > > +    unsigned long gp;
> > > > +    unsigned long tp;
> > > > +    unsigned long t0;
> > > > +    unsigned long t1;
> > > > +    unsigned long t2;
> > > > +    unsigned long s0;
> > > > +    unsigned long s1;
> > > > +    unsigned long a0;
> > > > +    unsigned long a1;
> > > > +    unsigned long a2;
> > > > +    unsigned long a3;
> > > > +    unsigned long a4;
> > > > +    unsigned long a5;
> > > > +    unsigned long a6;
> > > > +    unsigned long a7;
> > > > +    unsigned long s2;
> > > > +    unsigned long s3;
> > > > +    unsigned long s4;
> > > > +    unsigned long s5;
> > > > +    unsigned long s6;
> > > > +    unsigned long s7;
> > > > +    unsigned long s8;
> > > > +    unsigned long s9;
> > > > +    unsigned long s10;
> > > > +    unsigned long s11;
> > > > +    unsigned long t3;
> > > > +    unsigned long t4;
> > > > +    unsigned long t5;
> > > > +    unsigned long t6;
> > > > +    unsigned long sepc;
> > > > +    unsigned long sstatus;
> > > > +    /* pointer to previous stack_cpu_regs */
> > > > +    unsigned long pregs;
> > > > +};
> > > Just to restate what I said on the earlier version: We have a struct
> > > of
> > > this name in the public interface for x86. Besides to confusion about
> > > re-using the name for something private, I'd still like to understand
> > > what the public interface plans are. This is specifically because I
> > > think it would be better to re-use suitable public interface structs
> > > internally where possible. But that of course requires spelling out
> > > such parts of the public interface first.
> > I am not sure that I get you here.
> > I greped a little bit and found out that each architecture declares
> > this structure inside arch-specific folder.
> >
> > Mostly the usage of the cpu_user_regs is to save/restore current state
> > of CPU during traps ( exceptions/interrupts ) and context_switch().
> Arm effectively duplicates the public interface struct vcpu_guest_core_regs
> and the internal struct cpu_user_regs (and this goes as far as also
> duplicating the __DECL_REG() helper). Personally I find such duplication
> odd at the first glance at least; maybe there is a specific reason for this
> in Arm. But whether the public interface struct can be re-used can likely
> only be known once it was spelled out.
There are some force padding, a different ordering and some extra fields 
in the internal version to simplify the assembly code.
The rationale is explained in 1c38a1e937d3 ("xen: arm: separate guest 
user regs from internal guest state").
We also have a split between 32-bit and 64-bit to avoid doubling up the 
size in the former case.
Cheers,

--
Julien Grall