Re: [PATCH v1 13/17] xen/riscv: add unprivileged guest memory read helper

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.08.2026 17:36, Oleksii Kurochko wrote:
> On 8/12/26 5:30 PM, Jan Beulich wrote:
>> On 20.07.2026 18:02, Oleksii Kurochko wrote:
>>> Introduce riscv_vcpu_unpriv_read() to allow Xen to safely read guest memory
>>> using HLV/HLVX instructions while reliably capturing trap context.
>>
>> Both for the title and the function name: How does "unprivileged" matter 
>> here?
> 
> Unprivileged because HLV/HLVX reads guest memory as if it were accessed 
> from a less-privileged (guest) context, rather than by the hypervisor in 
> HS mode.
> 
> I think I am okay generally to drop "unpriv..." from the function name.
> 
>> The same functions would be use for reading Dom0's memory, wouldn't they?
> 
> Yes, I don't see any issue to let this function to read DomO's memory 
> too. But Dom0 could be counted as "unprivileged" too as it is executed 
> in VS-mode which is less-privileged then HS-mode.

Well, you need to properly distinguish the different cases of "unprivileged"
when writing titles / descriptions. The way you put it in your reply above
("less-privileged (guest)") is sufficiently unambiguous, but what you have
in the title ("unprivileged guest") clearly is a synonym for "DomU".

In the end, as you confirm, "guest" alone is all that's needed here to know
what is being talked about. (That said: We often distinguish "guest" and
"domain", with the former meaning DomU but the latter meaning all domains,
including Dom0 and service domains. Yet for the purpose here I think "guest"
is good to use, not matter that all kinds of domains are meant.)

>>> +    if ( read_insn )
>>> +    {
>>> +        asm volatile ( "\n"
>>> +            "1:\n"
>>> +            "   hlvx.hu %[val], (%[addr])\n"
>>> +            ASM_EXTABLE_TRAP_INFO(1b, 3f, %[ti])
>>> +            "   andi %[tmp], %[val], 3\n"
>>> +            "   addi %[tmp], %[tmp], -3\n"
>>> +            "   bne %[tmp], zero, 3f\n"
>>> +            "   addi %[addr], %[addr], 2\n"
>>> +            "\n"
>>> +            "2:\n"
>>> +            "   hlvx.hu %[tmp], (%[addr])\n"
>>> +            ASM_EXTABLE_TRAP_INFO(2b, 3f, %[ti])
>>> +            "   sll %[tmp], %[tmp], 16\n"
>>> +            "   add %[val], %[val], %[tmp]\n"
>>> +            "3:\n"
>>> +        : [val] "=&r" (val), [tmp] "=&r" (tmp), [addr] "+&r" (guest_addr)
>>> +        : [ti] "r" (trap) : "memory" );
>>
>> You want to tell the compiler that *trap is written. Instead I don't see
>> why a memory clobber would be needed: You access a different address space,
>> i.e. nothing the compiler can make any assumptions about.
> 
> memory clobber tells the compiler that the assembly code performs memory 
> reads or writes to items other than those listed in the input and output 
> operands and so I don't tell here that *trap will be changed.
> 
> Why this understanding is wrong?

You can (ab)use "memory" for that purpose, but why would you when you can
properly express the operand? All that achieves is the compiler possibly
having to emit less efficient code.

> Alternative, I think, could be:
>          : [val] "+r" (val), "+m" (*trap)
>          : [addr] "r" (guest_addr), [ti] "r" (trap) );
> And then memory clobber could be dropped.
> 
> 
> 
>>
>> You also need to take precautions for not returning an uninitialized "val".
>> I think the variable wants initializing (perhaps to ~0) and "+r" wants
>> using as constraint. (Afaik & isn't necessary to use together with +.)
> 
> I agree with '+' if we will initialize val with some value.
> 
> Regarding, '&' my understanding is that I have to use it always when

When what exactly? If an operand is both input and output, how could the
compiler re-use the (generally) register for any further purpose? '&'
indicates to the compiler that it may not use the register used for an
output to hold some input's value, as that value may be lost by the time
the input is actually consumed.

>>> +        /*
>>> +         * Although HLVX instructions' explicit memory accesses require 
>>> execute
>>> +         * permissions, they still raise the same exceptions as other load
>>> +         * instructions, rather than raising fetch exceptions instead.
>>> +         */
>>> +        if ( trap->scause == CAUSE_LOAD_PAGE_FAULT )
>>> +            trap->scause = CAUSE_FETCH_PAGE_FAULT;
>>> +    }
>>> +    else
>>> +    {
>>> +        asm volatile ( "\n"
>>> +            "1:\n"
>>> +#ifdef CONFIG_RISCV_64
>>> +            "hlv.d %[val], (%[addr])\n"
>>> +#else
>>> +            "hlv.w %[val], (%[addr])\n"
>>> +#endif
>>
>> Once again please use enough care that RV128 would at least obviously fail to
>> build, rather than building something which then doesn't work.
> 
> Sure, I will do the following:
> 
> #if defined(CONFIG_RISCV_64)
>              "hlv.d %[val], (%[addr])\n"
> #elif defined(CONFIG_RISCV_32)
>              "hlv.w %[val], (%[addr])\n"
> #else
>               #error "unsupported RISC-V variant: no hlv for a machine word"
> #endif

And that, as before, without indenting #, please.

Jan