linux-kernel - Re: [RFC PATCH 05/16] RISC-V: KVM: Implement VCPU interrupts and requests handling

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Message-ID: <9f9d09e5-49bc-f8e3-cfe1-bd5221e3b683@redhat.com>
Date:   Tue, 30 Jul 2019 13:17:26 +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 05/16] RISC-V: KVM: Implement VCPU interrupts and
 requests handling

First, something that is not clear to me: how do you deal with a guest
writing 1 to VSIP.SSIP?  I think that could lead to lost interrupts if
you have the following sequence

1) guest writes 1 to VSIP.SSIP

2) guest leaves VS-mode

3) host syncs VSIP

4) user mode triggers interrupt

5) host reenters guest

6) host moves irqs_pending to VSIP and clears VSIP.SSIP in the process

Perhaps irqs_pending needs to be split in two fields, irqs_pending and
irqs_pending_mask, and then you can do this:

/*
 * irqs_pending and irqs_pending_mask have multiple-producer/single-
 * consumer semantics; therefore bits can be set in the mask without
 * a lock, but clearing the bits requires vcpu_lock.  Furthermore,
 * consumers should never write to irqs_pending, and should not
 * use bits of irqs_pending that weren't 1 in the mask.
 */

int kvm_riscv_vcpu_set_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
{
	...
	set_bit(irq, &vcpu->arch.irqs_pending);
	smp_mb__before_atomic();
	set_bit(irq, &vcpu->arch.irqs_pending_mask);
	kvm_vcpu_kick(vcpu);
}

int kvm_riscv_vcpu_unset_interrupt(struct kvm_vcpu *vcpu, unsigned int irq)
{
	...
	clear_bit(irq, &vcpu->arch.irqs_pending);
	smp_mb__before_atomic();
	set_bit(irq, &vcpu->arch.irqs_pending_mask);
}

static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu)
{
	...
	WRITE_ONCE(vcpu->arch.irqs_pending_mask, 0);
}

and kvm_riscv_vcpu_flush_interrupts can leave aside VSIP bits that
aren't in vcpu->arch.irqs_pending_mask:

	if (atomic_read(&vcpu->arch.irqs_pending_mask)) {
		u32 mask, val;

		mask = xchg_acquire(&vcpu->arch.irqs_pending_mask, 0);
		val = READ_ONCE(vcpu->arch.irqs_pending) & mask;

		vcpu->arch.guest_csr.vsip &= ~mask;
		vcpu->arch.guest_csr.vsip |= val;
		csr_write(CSR_VSIP, vsip);
	}

Also, the getter of CSR_VSIP should call
kvm_riscv_vcpu_flush_interrupts, while the setter should clear
irqs_pending_mask.

On 29/07/19 13:56, Anup Patel wrote:
> +	kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> +	kvm_vcpu_kick(vcpu);

The request is not needed as long as kvm_riscv_vcpu_flush_interrupts is
called *after* smp_store_mb(vcpu->mode, IN_GUEST_MODE) in
kvm_arch_vcpu_ioctl_run.  This is the "request-less vCPU kick" pattern
in Documentation/virtual/kvm/vcpu-requests.rst.  The smp_store_mb then
orders the write of IN_GUEST_MODE before the read of irqs_pending (or
irqs_pending_mask in my proposal above); in the producers, there is a
dual memory barrier in kvm_vcpu_exiting_guest_mode(), ordering the write
of irqs_pending(_mask) before the read of vcpu->mode.

Similar to other VS* CSRs, I'd rather have a ONE_REG interface for VSIE
and VSIP from the beginning as well.  Note that the VSIP setter would
clear irqs_pending_mask, while the getter would call
kvm_riscv_vcpu_flush_interrupts before reading.  It's up to userspace to
ensure that no interrupt injections happen between the calls to the
getter and the setter.

Paolo

> +		csr_write(CSR_VSIP, vcpu->arch.irqs_pending);
> +		vcpu->arch.guest_csr.vsip = vcpu->arch.irqs_pending;
> +	}