Re: [PATCH v1 2/3] xen/riscv: add RISC-V legacy SBI extension support for guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.12.2025 13:52, Oleksii Kurochko wrote:
> On 12/11/25 12:02 PM, Jan Beulich wrote:
>> On 11.12.2025 11:29, Oleksii Kurochko wrote:
>>> On 12/8/25 4:05 PM, Jan Beulich wrote:
>>>> On 01.12.2025 11:24, Oleksii Kurochko wrote:
>>>>> --- /dev/null
>>>>> +++ b/xen/arch/riscv/vsbi/vsbi-legacy-extension.c
>>>>> @@ -0,0 +1,37 @@
>>>>> +
>>>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>>>> +
>>>>> +#include <xen/lib.h>
>>>>> +#include <xen/sched.h>
>>>>> +
>>>>> +#include <asm/processor.h>
>>>>> +#include <asm/vsbi.h>
>>>>> +
>>>>> +static int vsbi_legacy_ecall_handler(struct vcpu *vcpu, unsigned long 
>>>>> eid,
>>>>> +                                     unsigned long fid,
>>>>> +                                     struct cpu_user_regs *regs)
>>>>> +{
>>>>> +    int ret = 0;
>>>>> +
>>>>> +    switch ( eid )
>>>>> +    {
>>>>> +    case SBI_EXT_0_1_CONSOLE_PUTCHAR:
>>>>> +        printk("%c", (char)regs->a0);
>>>> This is guest output, so shouldn't use plain printk().
>>> I think that I don't know what should be used instead. Could you suggest me 
>>> something
>>> or point to the code in other arch-s?
>>>
>>> Or do you mean that guest_printk() should be used?
>> No direct replacement will do what you want, as they all prefix something to 
>> the
>> string passed (which isn't what you want). You may need to buffer characters 
>> and
>> emit them in batches (full lines unless overly long). For x86 see 
>> hvm_print_line(),
>> but I think Arm also has something like this.
> 
> I don’t recall anything like that for ARM. The only thing related to character
> buffering that I remember is in vpl011_write_data_xen()
> (https://elixir.bootlin.com/xen/v4.21.0/source/xen/arch/arm/vpl011.c#L76), 
> but it
> does not use the buf declared in struct domain_console. Instead, it provides a
> separate structure for vpl011:
>      struct vpl011_xen_backend {
>          char in[SBSA_UART_FIFO_SIZE];
>          char out[SBSA_UART_OUT_BUF_SIZE];
>          XENCONS_RING_IDX in_cons, in_prod;
>          XENCONS_RING_IDX out_prod;
>      };
> 
> I don’t see that ARM uses struct domain_console.
> 
> By the way, I can’t find a counterpart of hvm_print_line() for reading a 
> character(s).
> Is domain_console->buf intended to be used only for writing characters?

I don't think there's any particular intention, but of course you can use it
only for one of the two.

Jan