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Mon, 25 Aug 2025 09:22:12 -0700 (PDT) Date: Mon, 25 Aug 2025 11:22:11 -0500 From: Andrew Jones To: Jinyu Tang Cc: Anup Patel , Atish Patra , Conor Dooley , Yong-Xuan Wang , Paul Walmsley , Nutty Liu , kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] riscv: skip csr restore if vcpu preempted reload Message-ID: <20250825-69a3c8b588e0bb1fbb5b7beb@orel> References: <20250825121411.86573-1-tjytimi@163.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250825121411.86573-1-tjytimi@163.com> On Mon, Aug 25, 2025 at 08:14:11PM +0800, Jinyu Tang wrote: > The kvm_arch_vcpu_load() function is called in two cases for riscv: > 1. When entering KVM_RUN from userspace ioctl. > 2. When a preempted VCPU is scheduled back. > > In the second case, if no other KVM VCPU has run on this CPU since the > current VCPU was preempted, the guest CSR (including AIA CSRS and HGTAP) > values are still valid in the hardware and do not need to be restored. > > This patch is to skip the CSR write path when: > 1. The VCPU was previously preempted > (vcpu->scheduled_out == 1). > 2. It is being reloaded on the same physical CPU > (vcpu->arch.last_exit_cpu == cpu). > 3. No other KVM VCPU has used this CPU in the meantime > (vcpu == __this_cpu_read(kvm_former_vcpu)). > > This reduces many CSR writes with frequent preemption on the same CPU. > > Signed-off-by: Jinyu Tang > Reviewed-by: Nutty Liu > --- > v2 -> v3: > v2 was missing a critical check because I generated the patch from my > wrong (experimental) branch. This is fixed in v3. Sorry for my trouble. > > v1 -> v2: > Apply the logic to aia csr load. Thanks for > Andrew Jones's advice. > > arch/riscv/kvm/vcpu.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > Reviewed-by: Andrew Jones