linux-kernel - Re: [PATCH v2 4/7] kvm/mips: rework guest entry logic

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Date:   Thu, 20 Jan 2022 17:15:51 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, aleksandar.qemu.devel@...il.com,
        alexandru.elisei@....com, anup.patel@....com,
        aou@...s.berkeley.edu, atish.patra@....com,
        borntraeger@...ux.ibm.com, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        frankja@...ux.ibm.com, frederic@...nel.org, gor@...ux.ibm.com,
        hca@...ux.ibm.com, james.morse@....com, jmattson@...gle.com,
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        paulmck@...nel.org, paul.walmsley@...ive.com, peterz@...radead.org,
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        tglx@...utronix.de, tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH v2 4/7] kvm/mips: rework guest entry logic

On Thu, Jan 20, 2022 at 05:57:05PM +0100, Paolo Bonzini wrote:
> On 1/20/22 17:44, Mark Rutland wrote:
> > On Wed, Jan 19, 2022 at 10:58:51AM +0000, Mark Rutland wrote:
> > > In kvm_arch_vcpu_ioctl_run() we use guest_enter_irqoff() and
> > > guest_exit_irqoff() directly, with interrupts masked between these. As
> > > we don't handle any timer ticks during this window, we will not account
> > > time spent within the guest as guest time, which is unfortunate.
> > > 
> > > Additionally, we do not inform lockdep or tracing that interrupts will
> > > be enabled during guest execution, which caan lead to misleading traces
> > > and warnings that interrupts have been enabled for overly-long periods.
> > > 
> > > This patch fixes these issues by using the new timing and context
> > > entry/exit helpers to ensure that interrupts are handled during guest
> > > vtime but with RCU watching, with a sequence:
> > > 
> > > 	guest_timing_enter_irqoff();
> > > 
> > > 	guest_state_enter_irqoff();
> > > 	< run the vcpu >
> > > 	guest_state_exit_irqoff();
> > > 
> > > 	< take any pending IRQs >
> > > 
> > > 	guest_timing_exit_irqoff();
> > 
> > Looking again, this patch isn't sufficient.
> > 
> > On MIPS a guest exit will be handled by kvm_mips_handle_exit() *before*
> > returning into the "< run the vcpu >" step above, so we won't call
> > guest_state_exit_irqoff() before using RCU, etc.
> 
> Indeed.  kvm_mips_handle_exit has a weird mutual recursion through runtime-assembled code, but then this is MIPS...
> 
> This should do it:
> 
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index e59cb6246f76..6f2291f017f5 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -1192,6 +1192,7 @@ int kvm_mips_handle_exit(struct kvm_vcpu *vcpu)
>  	kvm_mips_set_c0_status();
>  	local_irq_enable();
> +	guest_timing_exit_irqoff();
>  	kvm_debug("kvm_mips_handle_exit: cause: %#x, PC: %p, kvm_run: %p, kvm_vcpu: %p\n",
>  			cause, opc, run, vcpu);
> @@ -1325,6 +1326,7 @@ int kvm_mips_handle_exit(struct kvm_vcpu *vcpu)
>  	}
>  	if (ret == RESUME_GUEST) {
> +		guest_timing_enter_irqoff();
>  		trace_kvm_reenter(vcpu);
>  		/*

As above, we'll also need the guest_state_{enter,exit}() calls
surrounding this (e.g. before that local_irq_enable() at the start of
kvm_mips_handle_exit(), and that needs to happen in noinstr code, etc.

It looks like the simplest thing to do is to rename
kvm_mips_handle_exit() to __kvm_mips_handle_exit() and add a
kvm_mips_handle_exit() wrapper that handles that (with the return path
conditional on whether __kvm_mips_handle_exit() returns RESUME_GUEST).

I'll have a go at that tomorrow when I regain the capability to think.

Longer-term MIPS should move to a loop like everyone else has:

	for (;;) {
		status = kvm_mips_enter_exit_vcpu();

		if (handle_exit(status))
			break;
		
		...
	}

... which is far easier to manage.

Thanks,
Mark.