Re: [PATCH v2 22/39] xen/riscv: add guest memory read helper

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

On 9/11/26 4:00 PM, Jan Beulich wrote:
> On 11.09.2026 15:57, Oleksii Kurochko wrote:
>>
>>
>> On 9/10/26 5:28 PM, Jan Beulich wrote:
>>> On 27.08.2026 17:21, Oleksii Kurochko wrote:
>>>> @@ -114,3 +115,89 @@ unsigned long copy_to_guest_phys(struct domain *d, paddr_t gpa, void *buf,
>>>>        return copy_guest(buf, gpa, len, GPA_INFO(d),
>>>>                          COPY_to_guest | COPY_gpa);
>>>>    }
>>>> +
>>>> +/*
>>>> + * Read machine word from guest memory
>>>> + *
>>>> + * @guest_addr: Guest address to read
>>>> + * @read_insn: Flag representing whether we are reading instruction
>>>> + * @trap: Output pointer to trap details if something went wrong during read
>>>> + *
>>>> + * The hlv/hlvx instructions translate guest_addr through the live
>>>> + * vsatp/hgatp CSRs, so the read is only meaningful for the address
>>>> + * space of the currently running vCPU.
>>>> + *
>>>> + * At most two halfwords are fetched when @read_insn is true, i.e. encodings
>>>> + * wider than 32 bits are not supported. Such an encoding cannot be completed
>>>> + * by calling this function again at @guest_addr + 4: the length check is
>>>> + * applied to the first halfword read, which would then be a continuation of
>>>> + * the instruction rather than its opcode. It is up to the caller to reject
>>>> + * anything that is neither a 16- nor a 32-bit encoding.
>>>> + */
>>>> +unsigned long riscv_read_guest(unsigned long guest_addr, bool read_insn,
>>>> +                               struct trap_info *trap)
>>>> +{
>>>> +    /*
>>>> +     * Poison the result: if the very first access faults, the fixup skips
>>>> +     * over the loads without writing it. Callers must check trap->scause.
>>>> +     */
>>>> +    unsigned long val = ~0UL, tmp;
>>>> +
>>>> +    /*
>>>> +     * hlv/hlvx use hstatus.SPVP for the privilege of the access, and the
>>>> +     * live vsatp/hgatp for the translation. Xen never installs a value of
>>>> +     * its own in hstatus (it is only saved on trap entry and restored
>>>> +     * before sret) and it doesn't reschedule before returning to the
>>>> +     * guest, so all three still belong to the vCPU which trapped.
>>>> +     *
>>>> +     * Check the saved copy rather than the live CSR: a nested trap taken
>>>> +     * from HS-mode clears hstatus.SPV in the CSR (but leaves SPVP alone).
>>>> +     */
>>>> +    ASSERT(vcpu_guest_cpu_user_regs(current)->hstatus & HSTATUS_SPV);
>>> [...]
>>> Question being of how much value
>>> that checking is: vcpu_guest_cpu_user_regs(current)->hstatus can't possibly
>>> have SPV clear, can it? Only nested exception frames could.
>>
>> Given that vcpu_guest_cpu_user_regs(current)->hstatus will always have
>> SPV set for any valid guest trap frame, the ASSERT is purely a defensive
>> sanity check to ensure riscv_read_guest() is never called outside a
>> guest trap context.
> 
> It is not, afaict: vcpu_guest_cpu_user_regs(current) will give you the guest
> frame no matter what context you're in. For what you want, you'd need to
> pass struct cpu_user_regs * into here.

Agree, struct cpu_user_regs * will be required.

But I am looking at how read_guest() is used and it shouldn't be used 
when nested HS trap happen never and it seems like it is too much to 
pass struct cpu_user_regs * just to check that.

I think it is enough just to have:

     case CAUSE_FETCH_GUEST_PAGE_FAULT:
     case CAUSE_LOAD_GUEST_PAGE_FAULT:
     case CAUSE_STORE_GUEST_PAGE_FAULT:
         /*
          * A guest page fault taken in Xen context comes from an hlv/hlvx
          * access made on a vCPU's behalf and is dealt with by the
          * fixup_exception() above, so only a guest can get here.
          */
         BUG_ON(!from_guest);

where we already checked that when riscv_read_guest() is used then it 
isn't nested HS trap.

So it looks like we could just drop the ASSERT() and probably update the 
comment above function and mention that riscv_read_guest() shouldn't be 
called in nested HS trap.

~ Oleksii