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

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Date:   Thu, 20 Jan 2022 17:57:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Cc:     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,
        joro@...tes.org, luto@...nel.org, maz@...nel.org, mingo@...hat.com,
        mpe@...erman.id.au, nsaenzju@...hat.com, palmer@...belt.com,
        paulmck@...nel.org, paul.walmsley@...ive.com, peterz@...radead.org,
        seanjc@...gle.com, suzuki.poulose@....com, svens@...ux.ibm.com,
        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 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);
  
  		/*


Paolo