linux-kernel - Re: [RFC PATCH v2 07/19] RISC-V: KVM: Implement KVM_GET_ONE_REG/KVM_SET_ONE

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Message-ID: <03f60f3a-bb50-9210-8352-da16cca322b9@redhat.com>
Date:   Fri, 2 Aug 2019 11:03:45 +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 07/19] RISC-V: KVM: Implement
 KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls

On 02/08/19 09:47, Anup Patel wrote:
> +	if (reg_num == KVM_REG_RISCV_CSR_REG(sip))
> +		kvm_riscv_vcpu_flush_interrupts(vcpu, false);

Not updating the vsip CSR here can cause an interrupt to be lost, if the
next call to kvm_riscv_vcpu_flush_interrupts finds a zero mask.

You could add a new field vcpu->vsip_shadow that is updated every time
CSR_VSIP is written (including kvm_arch_vcpu_load) with a function like

void kvm_riscv_update_vsip(struct kvm_vcpu *vcpu)
{
	if (vcpu->vsip_shadow != vcpu->arch.guest_csr.vsip) {
		csr_write(CSR_VSIP, vcpu->arch.guest_csr.vsip);
		vcpu->vsip_shadow = vcpu->arch.guest_csr.vsip;
	}
}

And just call this unconditionally from kvm_vcpu_ioctl_run.  The cost is
just a memory load per VS-mode entry, it should hardly be measurable.

Paolo