Re: [PATCH v7] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core

From: "Radim Krčmář" <radim.krcmar@oss.qualcomm.com>
To: "Jinyu Tang" <tjytimi@163.com>,
	"Anup Patel" <anup@brainfault.org>,
	"Atish Patra" <atish.patra@linux.dev>,
	"Andrew Jones" <andrew.jones@oss.qualcomm.com>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	"Yong-Xuan Wang" <yongxuan.wang@sifive.com>,
	"Nutty Liu" <nutty.liu@hotmail.com>,
	"Paul Walmsley" <pjw@kernel.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>
Cc: <kvm@vger.kernel.org>, <kvm-riscv@lists.infradead.org>,
	<linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v7] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core
Date: Fri, 27 Feb 2026 14:00:43 +0000	[thread overview]
Message-ID: <DGPSROCWMJWD.33FHG4JOPD2V8@oss.qualcomm.com> (raw)
In-Reply-To: <20260227121008.442241-1-tjytimi@163.com>

2026-02-27T20:10:08+08:00, Jinyu Tang <tjytimi@163.com>:
> Currently, kvm_arch_vcpu_load() unconditionally restores guest CSRs,
> HGATP, and AIA state. However, when a VCPU is loaded back on the same
> physical CPU, and no other KVM VCPU has run on this CPU since it was
> last put, the hardware CSRs and AIA registers are still valid.
>
> This patch optimizes the vcpu_load path by skipping the expensive CSR
> and AIA writes if all the following conditions are met:
> 1. It is being reloaded on the same CPU (vcpu->arch.last_exit_cpu == cpu).
> 2. The CSRs are not dirty (!vcpu->arch.csr_dirty).
> 3. No other VCPU used this CPU (vcpu == __this_cpu_read(kvm_former_vcpu)).
>
> To ensure this fast-path doesn't break corner cases:
> - Live migration and VCPU reset are naturally safe. KVM initializes
>   last_exit_cpu to -1, which guarantees the fast-path won't trigger.
> - The 'csr_dirty' flag tracks runtime userspace interventions. If
>   userspace modifies guest configurations (e.g., hedeleg via
>   KVM_SET_GUEST_DEBUG, or CSRs including AIA via KVM_SET_ONE_REG),
>   the flag is set to skip the fast path.
>
> With the 'csr_dirty' safeguard proven effective, it is safe to
> include kvm_riscv_vcpu_aia_load() inside the skip logic now.
>
> Signed-off-by: Jinyu Tang <tjytimi@163.com>
> ---
>  v6 -> v7:
>  - Moved kvm_riscv_vcpu_aia_load() into the fast-path skip logic, as
>    suggested by Radim Krčmář.
>  - Verified the fix for the IMSIC instability issue reported in v3.
>    Testing was conducted on QEMU 10.0.2 with explicitly enabled AIA
>    (`-machine virt,aia=aplic-imsic`). The guest boots successfully 
>    using virtio-mmio devices like virtio-blk and virtio-net.

Reviewed-by: Radim Krčmář <radim.krcmar@oss.qualcomm.com>

Thanks.

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