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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.09.2026 15:41, Oleksii Kurochko wrote:
> On 9/11/26 3:06 PM, Oleksii Kurochko wrote:
>> On 9/9/26 2:04 PM, Baptiste Le Duc wrote:
>>>> Introduce riscv_read_guest() to allow Xen to safely read guest memory
>>>> using HLV/HLVX instructions while reliably capturing trap context.
>>>
>>>> This is required for instruction fetch emulation and MMIO decoding, 
>>>> where
>>>> Xen must inspect guest memory that may not be directly accessible and 
>>>> may
>>>> fault.
>>>>
>>>> The implementation is based on kvm_riscv_vcpu_unpriv_read() from Linux,
>>>> with one deviation: the hlv/hlvx instructions translate the guest 
>>>> address
>>>> through the live vsatp/hgatp CSRs, i.e. through the address space of the
>>>> currently running vCPU, so the function can only be called safely for
>>>> current. Instead of taking a struct vcpu argument, it always operates on
>>>> current directly.
>>>>
>>>> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
>>>>
>>>> diff --git a/xen/arch/riscv/guestcopy.c b/xen/arch/riscv/guestcopy.c
>>>> index 8a89212e0b..b2327822ac 100644
>>>> --- a/xen/arch/riscv/guestcopy.c
>>>> +++ b/xen/arch/riscv/guestcopy.c
>>>> @@ -6,6 +6,7 @@
>>>>   #include <xen/string.h>
>>>>   #include <asm/guest_access.h>
>>>> +#include <asm/traps.h>
>>>>   #define COPY_from_guest     0U
>>>>   #define COPY_to_guest       BIT(0, U)
>>>> @@ -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)
>>> Nit: every other function in this file / declared in this header
>>> (raw_copy_from_guest, copy_to_guest_phys, ...) has no riscv_ prefix. Why
>>> does this one get it?
>>
>> Agree not to much sense. I will drop it.
> 
> Probably we still want to have riscv_ prefix to show that this 
> read_guest() is specific to RISC-V but others (raw_copy_from_guest, 
> copy_to_guest_phys, ...) could be used in Xen common code too.
> 
> So I think we could keep riscv_ prefix.

Please may I suggest to avoid unnecessary prefixes?

Jan