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

[Jan Beulich <jbeulich@xxxxxxxx>]

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. That said, seeing
that Andrew had asked for this, so be it then (albeit I remain unconvinced).

Jan