linux-kernel - Re: [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

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Message-ID: <933ca564-973d-645e-fe9c-9afb64edba5b@redhat.com>
Date:   Tue, 15 Oct 2019 10:28:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers

On 28/09/19 19:23, Andrea Arcangeli wrote:
> Reducing this list to only EXIT_REASON_MSR_WRITE,
> EXIT_REASON_PREEMPTION_TIMER, EXIT_REASON_EPT_MISCONFIG,
> EXIT_REASON_IO_INSTRUCTION increases the computation time of the
> hrtimer guest testcase on Haswell i5-4670T CPU @ 2.30GHz by 7% with
> the default spectre v2 mitigation enabled in the host and guest. On
> skylake as opposed there's no measurable difference with the short
> list. To put things in prospective on Haswell the same hrtimer
> workload (note: it never calls cpuid and it never attempts to trigger
> more vmexit on purpose) in guest takes 16.3% longer to compute on
> upstream KVM running in the host than with the KVM mono v1 patchset
> applied to the host kernel, while on skylake the same takes only 5.4%
> more time (both with the default mitigations enabled in guest and
> host).
> 
> It's also unclear why EXIT_REASON_IO_INSTRUCTION should be included.

If you're including EXIT_REASON_EPT_MISCONFIG (MMIO access) then you
should include EXIT_REASON_IO_INSTRUCTION too.  Depending on the devices
that are in the guest, the doorbell register might be MMIO or PIO.

> +		if (exit_reason == EXIT_REASON_MSR_WRITE)
> +			return kvm_emulate_wrmsr(vcpu);
> +		else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER)
> +			return handle_preemption_timer(vcpu);
> +		else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT)
> +			return handle_interrupt_window(vcpu);
> +		else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
> +			return handle_external_interrupt(vcpu);
> +		else if (exit_reason == EXIT_REASON_HLT)
> +			return kvm_emulate_halt(vcpu);
> +		else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION)
> +			return handle_pause(vcpu);
> +		else if (exit_reason == EXIT_REASON_MSR_READ)
> +			return kvm_emulate_rdmsr(vcpu);
> +		else if (exit_reason == EXIT_REASON_CPUID)
> +			return kvm_emulate_cpuid(vcpu);
> +		else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
> +			return handle_ept_misconfig(vcpu);

So, the difference between my suggested list (which I admit is just
based on conjecture, not benchmarking) is that you add
EXIT_REASON_PAUSE_INSTRUCTION, EXIT_REASON_PENDING_INTERRUPT,
EXIT_REASON_EXTERNAL_INTERRUPT, EXIT_REASON_HLT, EXIT_REASON_MSR_READ,
EXIT_REASON_CPUID.

Which of these make a difference for the hrtimer testcase?  It's of
course totally fine to use benchmarks to prove that my intuition was
bad---but you must also use them to show why your intuition is right. :)

Paolo