linux-kernel - Re: [PATCH v2] x86: svm: use kvm_fast_pio

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Date:	Mon, 2 Mar 2015 19:49:54 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Joel Schopp <joel.schopp@....com>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	David Kaplan <David.Kaplan@....com>,
	Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2] x86: svm: use kvm_fast_pio_in()

2015-03-02 10:40-0600, Joel Schopp:
> From: David Kaplan <David.Kaplan@....com>
> 
> We can make the in instruction go faster the same way the out instruction is
> already.
> 
> Changes from v1
> 	* Added kvm_fast_pio_in() implementation that was left out of v1
> 
> Signed-off-by: David Kaplan <David.Kaplan@....com>
> [extracted from larger unlrelated patch, forward ported, tested]
> Signed-off-by: Joel Schopp <joel.schopp@....com>
> ---
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index d319e0c..f8c906b 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1899,7 +1899,7 @@ static int io_interception(struct vcpu_svm *svm)
>  	++svm->vcpu.stat.io_exits;
>  	string = (io_info & SVM_IOIO_STR_MASK) != 0;
>  	in = (io_info & SVM_IOIO_TYPE_MASK) != 0;
> -	if (string || in)
> +	if (string)

(I guess that most accesses are covered now, so we don't need to make
 REP case a bit faster ...)

>  		return emulate_instruction(vcpu, 0) == EMULATE_DONE;
>  
>  	port = io_info >> 16;
> @@ -1907,6 +1907,8 @@ static int io_interception(struct vcpu_svm *svm)
>  	svm->next_rip = svm->vmcb->control.exit_info_2;
>  	skip_emulated_instruction(&svm->vcpu);
>  
> +	if (in)
> +		return kvm_fast_pio_in(vcpu, size, port);
>  	return kvm_fast_pio_out(vcpu, size, port);

(kvm_fast_pio() comes to mind.)

>  }
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bd7a70b..089247c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5463,6 +5463,39 @@ int kvm_fast_pio_out(struct kvm_vcpu *vcpu, int size, unsigned short port)
>  }
>  EXPORT_SYMBOL_GPL(kvm_fast_pio_out);
>  
> +static int complete_fast_pio(struct kvm_vcpu *vcpu)
> +{
> +	u32 new_rax = kvm_register_read(vcpu, VCPU_REGS_RAX);

u64.

> +
> +	if (!vcpu->arch.pio.count)
> +		return 0;
> +	if (vcpu->arch.pio.count * vcpu->arch.pio.size > 8)
> +		return 0;

sizeof(new_rax).  (safer and easier to understand)

Both should never happen in KVM code, BUG_ON().

> +
> +	memcpy(&new_rax, vcpu->arch.pio_data,
> +	       vcpu->arch.pio.count * vcpu->arch.pio.size);

Use emulator_pio_in_emulated() here, for code sharing.
(We want to trace the read here too;  it could be better to split
 the path from emulator_pio_in_emulated() first.)

> +	kvm_register_write(vcpu, VCPU_REGS_RAX, new_rax);
> +
> +	vcpu->arch.pio.count = 0;
> +	return 1;
> +}
> +
> +int kvm_fast_pio_in(struct kvm_vcpu *vcpu, int size, unsigned short port)
> +{
> +	unsigned long val;
> +	int ret = emulator_pio_in_emulated(&vcpu->arch.emulate_ctxt, size,
> +					   port, &val, 1);
> +
> +	if (ret) {
> +		kvm_register_write(vcpu, VCPU_REGS_RAX, val);
> +		vcpu->arch.pio.count = 0;

(emulator_pio_in_emulated() sets count to zero if it returns true.)

> +	} else
> +		vcpu->arch.complete_userspace_io = complete_fast_pio;
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(kvm_fast_pio_in);
> +
>  static void tsc_bad(void *info)
>  {
>  	__this_cpu_write(cpu_tsc_khz, 0);
> 
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