Re: [PATCH v1 15/17] xen/riscv: implement trap redirection to a guest

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.08.2026 09:47, Oleksii Kurochko wrote:
> On 8/12/26 6:03 PM, Jan Beulich wrote:
>> On 20.07.2026 18:02, Oleksii Kurochko wrote:
>>> Some traps taken by Xen on behalf of a guest can't or shouldn't be
>>> handled by the hypervisor and must be forwarded to the guest's own
>>> S-mode exception handler instead: e.g. when riscv_vcpu_unpriv_read()
>>> faults while accessing guest memory, or when emulation hits a condition
>>> only the guest kernel can resolve.
>>
>> Is the plan to use riscv_vcpu_unpriv_read() also for reading hypercall
>> buffers?
> 
> Yes, it could also be used to read hypercall buffers, but I don't think 
> it's the best option, as hypercall buffers could be larger than 8 bytes 
> (which is the size supported by the `hlv` instruction on the RV64 
> platform). For that case, I think it would be better to map the Xen page 
> corresponding to the GVA of the hypercall buffer and then use the usual 
> memcpy(). So, basically, use copy_guest() on RISC-V for that purpose.
> 
> 
>> In that case trap redirection shouldn't come into play.
> 
> It isn't mandatory to perform a redirection in the case of 
> riscv_vcpu_unpriv_read(), so if trap redirection shouldn't happen for 
> hypercall buffers, then the caller of riscv_vcpu_unpriv_read() needs to 
> handle that properly by checking utrap.cause. Something like:
> 
> ```
>      *insn = riscv_vcpu_unpriv_read(true, regs->sepc, &utrap);
>      if ( utrap.scause )
>      {
>          ...
>          utrap.sepc = regs->sepc;
>          utrap.stval = utrap.sepc;
> 
>          riscv_vcpu_trap_redirect(&utrap);
> 
>          return true;
>      }
> ```
> 
> So, if this cannot happen in the case of a hypercall buffer, then we 
> need to return -EFAULT in the if ( utrap.scause ) case.
> 
> I don't think I understand why redirection shouldn't come into play. Do 
> you mean that the hypercall buffer will always be available, and that it 
> is impossible for the hlv instruction to fail, so there is no point in 
> handling redirection at all in this case?

Failure to access a hypercall buffer should result in a -EFAULT return
value, not in any kind of exception.

Jan