linux-kernel - Re: [PATCH v2] RISC-V: KVM: Delegate kvm unhandled faults to VS mode

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Message-Id: <DB908AUW6N76.3SYAGIFGCDJ27@ventanamicro.com>
Date: Fri, 11 Jul 2025 08:16:22 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Xu Lu" <luxu.kernel@...edance.com>, <cleger@...osinc.com>,
 <anup@...infault.org>, <atish.patra@...ux.dev>, <paul.walmsley@...ive.com>,
 <palmer@...belt.com>, <aou@...s.berkeley.edu>, <alex@...ti.fr>
Cc: <kvm@...r.kernel.org>, <kvm-riscv@...ts.infradead.org>,
 <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] RISC-V: KVM: Delegate kvm unhandled faults to VS
 mode

2025-07-10T21:30:30+08:00, Xu Lu <luxu.kernel@...edance.com>:
> Delegate faults which are not handled by kvm to VS mode to avoid
> unnecessary traps to HS mode. These faults include illegal instruction
> fault, instruction access fault, load access fault and store access
> fault.
>
> The delegation of illegal instruction fault is particularly important
> to guest applications that use vector instructions frequently. In such
> cases, an illegal instruction fault will be raised when guest user thread
> uses vector instruction the first time and then guest kernel will enable
> user thread to execute following vector instructions.

(This optimization will be even more significant when nesting, where it
 would currently go -> HS0 -> HS1 -> HS0 -> VS1, instead of -> VS1.)

> The fw pmu event counters remain undeleted so that guest can still get
> these events via sbi call. Guest will only see zero count on these
> events and know 'firmware' has delegated these faults.
>
> Signed-off-by: Xu Lu <luxu.kernel@...edance.com>
> ---

Reviewed-by: Radim Krčmář <rkrcmar@...tanamicro.com>