Re: [PATCH v1 01/15] xen/riscv: introduce struct arch_vcpu

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 1/6/26 4:33 PM, Jan Beulich wrote:
> On 06.01.2026 16:05, Oleksii Kurochko wrote:
> > On 1/6/26 3:26 PM, Jan Beulich wrote:
> > > On 06.01.2026 15:19, Oleksii Kurochko wrote:
> > > > On 1/5/26 5:58 PM, Jan Beulich wrote:
> > > > > On 24.12.2025 18:03, Oleksii Kurochko wrote:
> > > > > > Introduce structure with VCPU's registers which describes its state.
> > > > > >
> > > > > > Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> > > > > Since none of this is being used for the time being, I think the description
> > > > > wants to be a little less terse. Coming from the x86 (rather then the Arm)
> > > > > side, I find the arrangements irritating. And even when comparing to Arm, ...
> > > > >
> > > > > > --- a/xen/arch/riscv/include/asm/domain.h
> > > > > > +++ b/xen/arch/riscv/include/asm/domain.h
> > > > > > @@ -22,9 +22,63 @@ struct hvm_domain
> > > > > >     struct arch_vcpu_io {
> > > > > >     };
> > > > > >     
> > > > > > -struct arch_vcpu {
> > > > > > +struct arch_vcpu
> > > > > > +{
> > > > > >         struct vcpu_vmid vmid;
> > > > > > -};
> > > > > > +
> > > > > > +    /* Xen's state: Callee-saved registers and tp, gp, ra */
> > > > > ... I don't think the following structure describes "Xen's state". On Arm
> > > > > it's guest controlled register values which are being saved afaict. I
> > > > > would then expect the same to become the case for RISC-V.
> > > > I think this is not fully correct, because guest-controlled registers on
> > > > Arm are allocated on the stack [1][2].
> > > I'll admit that I should have said "possibly guest-controlled". Callee-
> > > saved registers may or may not be used in functions, and if one isn't
> > > used throughout the call-stack reaching __context_switch(), it would
> > > still hold whatever the guest had put there.
> > But the guest doesn't put there nothing, only Xen does that and it is a reason
> > why I am trying to call it Xen state. Guest works only with what is stored in
> > struct cpu_info->guest_cpu_user_regs.* ...
> >
> > > > Regarding|xen_saved_context| (or|saved_context| on Arm, which I used as a base),
> > > > I think|xen_saved_context| is a slightly better name. Looking at how the
> > > > |saved_context| structure is used on Arm [3], it can be concluded that
> > > > |__context_switch()| switches only Xen’s internal context. What actually 
> > > > happens is
> > > > that|__context_switch()| is called while running on the previous vCPU’s stack
> > > > and returns on the next vCPU’s stack. Therefore, it is necessary to have
> > > > the correct register values stored in the|saved_context| structure in order
> > > > to continue Xen’s execution when it later returns to the previous stack.
> > > For this and ...
> > >
> > > > Probably I need to introduce|__context_switch()| in this patch series for RISC-V
> > > > now; I hope this will clarify things better. At the moment, it looks like [4].
> > > >
> > > > [1] 
> > > > https://elixir.bootlin.com/xen/v4.21.0/source/xen/arch/arm/include/asm/arm64/processor.h#L14
> > > > [2] https://elixir.bootlin.com/xen/v4.21.0/source/xen/arch/arm/domain.c#L547
> > > >
> > > > [3] 
> > > > https://elixir.bootlin.com/xen/v4.21.0/source/xen/arch/arm/arm64/entry.S#L650
> > > >
> > > > [4] 
> > > > https://gitlab.com/xen-project/people/olkur/xen/-/blob/riscv-next-upstreaming/xen/arch/riscv/entry.S?ref_type=heads#L153
> > > >
> > > > > > +    struct
> > > > > > +    {
> > > > > > +        register_t s0;
> > > > > > +        register_t s1;
> > > > > > +        register_t s2;
> > > > > > +        register_t s3;
> > > > > > +        register_t s4;
> > > > > > +        register_t s5;
> > > > > > +        register_t s6;
> > > > > > +        register_t s7;
> > > > > > +        register_t s8;
> > > > > > +        register_t s9;
> > > > > > +        register_t s10;
> > > > > > +        register_t s11;
> > > > > > +
> > > > > > +        register_t sp;
> > > > > > +        register_t gp;
> > > > > > +
> > > > > > +        /* ra is used to jump to guest when creating new vcpu */
> > > > > > +        register_t ra;
> > > > > > +    } xen_saved_context;
> > > > > The xen_ prefix here also doesn't exist in Arm code.
> > > > I think it should be added for Arm too. I can send a patch.
> > > ... this, to reword my comment: What value does the xen_ prefix add?
> > ... because guest doesn't access saved_context and as I mentioned above
> > guest has "access" only to struct cpu_info->guest_cpu_user_regs.*.
> The guest has no access to anything in the hypervisor.
Of course, the guest doesn't have access. By "access" I meant guest context
stored in cpu_info->guest_cpu_user_regs.*.

>   That said, seeing
> that Andrew had asked for this, so be it then (albeit I remain unconvinced).
I will add some extra comments about xen_saved_context to make things more
clear.

~ Oleksii