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

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Message-Id: <DATTT5U64J4L.3UTDRVT2YP7GT@ventanamicro.com>
Date: Mon, 23 Jun 2025 12:04:30 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Xu Lu" <luxu.kernel@...edance.com>
Cc: <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

2025-06-22T18:11:49+08:00, Xu Lu <luxu.kernel@...edance.com>:
> Hi Radim,
>
> On Fri, Jun 20, 2025 at 8:04 PM Radim Krčmář <rkrcmar@...tanamicro.com> 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.
>
> I only want to delegate it by default. And KVM may still want to
> delegate different exceptions for different VMs like what it does for
> EXC_BREAKPOINT.

(I think we could easily reintroduce the code if KVM wants to do that in
 the future.  I also think that it's bad that this patch is doing an
 observable change without userspace involvement -- the counting of KVM
 SBI PMU events, but others will probably disagree with me on this.)

>                 So maybe it is better to reserve these codes?

Possibly, the current is acceptable if you have considered the
implications on PMU events.

>> And why not delegate the others as well?
>> (EXC_LOAD_MISALIGNED, EXC_STORE_MISALIGNED, EXC_LOAD_ACCESS,
>>  EXC_STORE_ACCESS, and EXC_INST_ACCESS.)
>
> Thanks for the reminder. I will have a test and resend the patch if it works.

The misaligned exceptions are already being worked on, so don't waste
your time on them, sorry.