Re: [External] Re: [PATCH] RISC-V: KVM: Delegate illegal instruction fault

["Clément Léger" <cleger@rivosinc.com>]

On 23/06/2025 15:30, Xu Lu wrote:
> Hi Clément,
> 
> On Mon, Jun 23, 2025 at 8:35 PM Clément Léger <cleger@rivosinc.com> wrote:
>>
>>
>>
>> On 23/06/2025 14:12, Xu Lu wrote:
>>> Hi Clément,
>>>
>>> On Mon, Jun 23, 2025 at 4:05 PM Clément Léger <cleger@rivosinc.com> wrote:
>>>>
>>>>
>>>>
>>>> On 20/06/2025 14:04, Radim Krčmář wrote:
>>>>> 2025-06-20T17:17:20+08:00, Xu Lu <luxu.kernel@bytedance.com>:
>>>>>> Delegate illegal instruction fault to VS mode in default to avoid such
>>>>>> exceptions being trapped to HS and redirected back to VS.
>>>>>>
>>>>>> Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
>>>>>> ---
>>>>>> diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
>>>>>> @@ -48,6 +48,7 @@
>>>>>> +                                     BIT(EXC_INST_ILLEGAL)    | \
>>>>>
>>>>> You should also remove the dead code in kvm_riscv_vcpu_exit.
>>>>>
>>>>> And why not delegate the others as well?
>>>>> (EXC_LOAD_MISALIGNED, EXC_STORE_MISALIGNED, EXC_LOAD_ACCESS,
>>>>>  EXC_STORE_ACCESS, and EXC_INST_ACCESS.)
>>>>
>>>> Currently, OpenSBI does not delegate misaligned exception by default and
>>>> handles misaligned access by itself, this is (partially) why we added
>>>> the FWFT SBI extension to request such delegation. Since some supervisor
>>>> software expect that default, they do not have code to handle misaligned
>>>> accesses emulation. So they should not be delegated by default.
>>>
>>> It doesn't matter whether these exceptions are delegated in medeleg.
>>
>> Not sure to totally understand, but if the exceptions are not delegated
>> in medeleg, they won't be delegated to VS-mode even though hedeleg bit
>> is set right ? The spec says:
>>
>> A synchronous trap that has been delegated to HS-mode (using medeleg)
>> is further delegated to VS-mode if V=1 before the trap and the
>> corresponding hedeleg bit is set.
> 
> Yes, you are right. The illegal insn exception is still trapped in M
> mode if it is not delegated in medeleg. But delegating it in hedeleg
> is still useful. The opensbi will check CSR_HEDELEG in the function
> sbi_trap_redirect. If the exception has been delegated to VS-mode in
> CSR_HEDLEG, opensbi can directly return back to VS-mode, without the
> overhead of going back to HS-mode and then going back to VS-mode.
> 
>>
>>
>>
>>> KVM in HS-mode does not handle illegal instruction or misaligned
>>> access and only redirects them back to VS-mode. Delegating such
>>> exceptions in hedeleg helps save CPU usage even when they are not
>>> delegated in medeleg: opensbi will check whether these exceptions are
>>> delegated to VS-mode and redirect them to VS-mode if possible. There
>>> seems to be no conflicts with SSE implementation. Please correct me if
>>> I missed anything.
>>
>> AFAIU, this means that since medeleg bit for misaligned accesses were
>> not delegated up to the introduction of the FWFT extension, VS-mode
>> generated misaligned accesses were handled by OpenSBI right ? Now that
>> we are requesting openSBI to delegate misaligned accesses, HS-mode
>> handles it's own misaligned accesses through the trap handler. With that
>> configuration, if VS-mode generate a misaligned access, it will end up
>> being redirected to VS-mode and won't be handle by HS-mode.
>>
>> To summarize, prior to FWFT, medeleg wasn't delegating misaligned
>> accesses to S-mode:
>>
>> - VS-mode misaligned access -> trap to M-mode -> OpenSBI handle it ->
>> Back to VS-mode, misaligned access fixed up by OpenSBI
> 
> Yes, this is what I want the procedure of handling illegal insn
> exceptions to be. Actually it now behaves as:
> 
> VS-mode illegal insn exception -> trap to M-mode -> OpenSBI handles it
> -> Back to HS-mode, does nothing -> Back to VS-mode.
> 
> I want to avoid going through HS-mode.

Hi Xu,

Yeah, that make sense as well but that should only happen if the VS-mode
requested misaligned access delegation via KVM SBI FWFT interface. I
know that currently KVM does do anything useful from the misaligned
exception except redirecting it to VS-mode but IMHO, that's a regression
I introduced with FWFT misaligned requested delegation...

> 
>>
>> Now that Linux uses SBI FWFT to delegates misaligned accesses (without
>> hedeleg being set for misaligned delegation, but that doesn't really
>> matter, the outcome is the same):
>>
>> - VS-mode misaligned access -> trap to HS-mode -> redirection to
>> VS-mode, needs to handle the misaligned access by itself
>>
>>
>> This means that previously, misaligned access were silently fixed up by
>> OpenSBI for VS-mode and now that FWFT is used for delegation, this isn't
>> true anymore. So, old kernel or sueprvisor software that  included code
>> to handle misaligned accesses will crash. Did I missed something ?
> 
> Great! You make it very clear! Thanks for your explanation. But even
> when misalign exceptions are delegated to HS-mode, KVM seems to do
> nothing but redirect to VS-mode when VM get trapped due to misalign
> exceptions. 

Exactly, which is why I said that either setting hedeleg by default or
not will lead to the same outcome, ie: VS-mode needs to handle access by
itself (which is a regression introduced by FWFT usage).


> So maybe we can directly delegate the misaligned
> exceptions in hedeleg too before running VCPU and retrieve them after
> VCPU exists. And then the handling procedure will be:
> 
> VS-mode misaligned exception -> trap to VS-mode -> VS handles it ->
> Back to VU-mode.

I'd better want to let the HS-mode handle the misaligned accesses if not
requested via the KVM SBI FWFT interface by VS-mode to keep HS-mode
expected behavior. As you pointed out, this is not currently the case
and the misaligned exceptions are directly redirected to VS-mode, this
differs from what was actually done previously without FWFT (ie OpenSBI
handles the misaligned access).

To summarize, I think HS-mode should fixup VS-mode misaligned accesses
unless requested via KVM SBI FWFT interface, in which case it will
delegates them (which is done by the FWFT series). This would match the
HS-mode <-> OpenSBI behavior.

Thanks,

Clément

> 
> Please correct me if I missed anything.
> 
> Best Regards,
> 
> Xu Lu
> 
>>
>> Note: this is not directly related to your series but my introduction of
>> FWFT !
>>
>> Thanks,
>>
>> Clément
>>
>>>
>>> Best Regards,
>>> Xu Lu
>>>
>>>>
>>>> Thanks,
>>>>
>>>> Clément
>>>>
>>>>>
>>>>> Thanks.
>>>>>
>>>>> _______________________________________________
>>>>> linux-riscv mailing list
>>>>> linux-riscv@lists.infradead.org
>>>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>>>
>>


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