linux-kernel - Re: [RFC PATCH v2 08/19] RISC-V: KVM: Implement VCPU world-switch

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Date:   Fri, 2 Aug 2019 10:30:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Anup Patel <anup@...infault.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 08/19] RISC-V: KVM: Implement VCPU world-switch

On 02/08/19 09:47, Anup Patel wrote:
> +	/* Save Host SSTATUS, HSTATUS, SCRATCH and STVEC */
> +	csrr	t0, CSR_SSTATUS
> +	REG_S	t0, (KVM_ARCH_HOST_SSTATUS)(a0)
> +	csrr	t1, CSR_HSTATUS
> +	REG_S	t1, (KVM_ARCH_HOST_HSTATUS)(a0)
> +	csrr	t2, CSR_SSCRATCH
> +	REG_S	t2, (KVM_ARCH_HOST_SSCRATCH)(a0)
> +	csrr	t3, CSR_STVEC
> +	REG_S	t3, (KVM_ARCH_HOST_STVEC)(a0)
> +

A possible optimization: if these cannot change while Linux runs (I am
thinking especially of STVEC and HSTATUS, but perhaps SSCRATCH can be
saved on kvm_arch_vcpu_load too) you can avoid the csrr and store.

Paolo