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

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Message-ID: <CAPYmKFs7tmMg4VQX=5YFhSzDGxodiBxv+v1SoqwTHvE1Khsr_A@mail.gmail.com>
Date: Mon, 23 Jun 2025 20:12:31 +0800
From: Xu Lu <luxu.kernel@...edance.com>
To: Clément Léger <cleger@...osinc.com>
Cc: Radim Krčmář <rkrcmar@...tanamicro.com>, 
	anup@...infault.org, atish.patra@...ux.dev, paul.walmsley@...ive.com, 
	palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr, kvm@...r.kernel.org, 
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, 
	linux-riscv <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [External] Re: [PATCH] RISC-V: KVM: Delegate illegal instruction fault

Hi Clément,

On Mon, Jun 23, 2025 at 4:05 PM Clément Léger <cleger@...osinc.com> wrote:
>
>
>
> On 20/06/2025 14:04, Radim Krčmář wrote:
> > 2025-06-20T17:17:20+08:00, Xu Lu <luxu.kernel@...edance.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@...edance.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.
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.

Best Regards,
Xu Lu

>
> Thanks,
>
> Clément
>
> >
> > Thanks.
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>

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