linux-kernel - Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath

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Date:   Thu, 14 Nov 2019 12:58:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath

Ok, it's not _so_ ugly after all.

> ---
>  arch/x86/kvm/vmx/vmx.c   | 39 +++++++++++++++++++++++++++++++++++++--
>  include/linux/kvm_host.h |  1 +
>  2 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 5d21a4a..5c67061 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5924,7 +5924,9 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> -	if (exit_reason < kvm_vmx_max_exit_handlers
> +	if (vcpu->fast_vmexit)
> +		return 1;
> +	else if (exit_reason < kvm_vmx_max_exit_handlers

Instead of a separate vcpu->fast_vmexit, perhaps you can set exit_reason
to vmx->exit_reason to -1 if the fast path succeeds.

> +			if (ret == 0)
> +				ret = kvm_skip_emulated_instruction(vcpu);

Please move the "kvm_skip_emulated_instruction(vcpu)" to
vmx_handle_exit, so that this basically is

#define EXIT_REASON_NEED_SKIP_EMULATED_INSN -1

	if (ret == 0)
		vcpu->exit_reason = EXIT_REASON_NEED_SKIP_EMULATED_INSN;

and handle_ipi_fastpath can return void.

Thanks,

Paolo

> +		};
> +	};
> +
> +	return ret;
> +}
> +
>  static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -6615,6 +6645,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  				  | (1 << VCPU_EXREG_CR3));
>  	vcpu->arch.regs_dirty = 0;
>  
> +	vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> +	vcpu->fast_vmexit = false;
> +	if (!is_guest_mode(vcpu) &&
> +		vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> +		vcpu->fast_vmexit = handle_ipi_fastpath(vcpu);

This should be done later, at least after kvm_put_guest_xcr0, because
running with partially-loaded guest state is harder to audit.  The best
place to put it actually is right after the existing vmx->exit_reason
assignment, where we already handle EXIT_REASON_MCE_DURING_VMENTRY.

>  	pt_guest_exit(vmx);
>  
>  	/*
> @@ -6634,7 +6670,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	vmx->nested.nested_run_pending = 0;
>  	vmx->idt_vectoring_info = 0;
>  
> -	vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
>  	if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
>  		kvm_machine_check();
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 719fc3e..7a7358b 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -319,6 +319,7 @@ struct kvm_vcpu {
>  #endif
>  	bool preempted;
>  	bool ready;
> +	bool fast_vmexit;
>  	struct kvm_vcpu_arch arch;
>  	struct dentry *debugfs_dentry;
>  };
>