linux-kernel - Re: [PATCH 3/6] KVM: x86: wean in-kernel PIO from vcpu->arch.pio*

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Date:   Thu, 9 Jun 2022 22:19:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 3/6] KVM: x86: wean in-kernel PIO from vcpu->arch.pio*

On Wed, Jun 08, 2022, Paolo Bonzini wrote:
>  static void complete_emulator_pio_in(struct kvm_vcpu *vcpu, void *val)
> @@ -7482,16 +7486,11 @@ static int emulator_pio_in(struct kvm_vcpu *vcpu, int size,
>  		 * shenanigans as KVM doesn't support modifying the rep count,
>  		 * and the emulator ensures @count doesn't overflow the buffer.
>  		 */
> +		complete_emulator_pio_in(vcpu, val);
> +		return 1;
>  	} else {
> -		int r = __emulator_pio_in(vcpu, size, port, count);
> -		if (!r)
> -			return r;
> -
> -		/* Results already available, fall through.  */
> +		return __emulator_pio_in(vcpu, size, port, val, count);

Any objections to not using an "else"?  I.e.

	if (vcpu->arch.pio.count) {
		/*
		 * Complete a previous iteration that required userspace I/O.
		 * Note, @count isn't guaranteed to match pio.count as userspace
		 * can modify ECX before rerunning the vCPU.  Ignore any such
		 * shenanigans as KVM doesn't support modifying the rep count,
		 * and the emulator ensures @count doesn't overflow the buffer.
		 */
		complete_emulator_pio_in(vcpu, val);
		return 1;
	}
	return __emulator_pio_in(vcpu, size, port, val, count);

>  	}
> -
> -	complete_emulator_pio_in(vcpu, val);
> -	return 1;
>  }
>  
>  static int emulator_pio_in_emulated(struct x86_emulate_ctxt *ctxt,
> @@ -7506,14 +7505,8 @@ static int emulator_pio_out(struct kvm_vcpu *vcpu, int size,
>  			    unsigned short port, const void *val,
>  			    unsigned int count)
>  {
> -	int ret;
> -
>  	trace_kvm_pio(KVM_PIO_OUT, port, size, count, val);
> -	ret = emulator_pio_in_out(vcpu, size, port, (void *)val, count, false);
> -	if (ret)
> -                vcpu->arch.pio.count = 0;
> -
> -        return ret;
> +	return emulator_pio_in_out(vcpu, size, port, (void *)val, count, false);
>  }
>  
>  static int emulator_pio_out_emulated(struct x86_emulate_ctxt *ctxt,
> @@ -13064,20 +13057,20 @@ static int kvm_sev_es_outs(struct kvm_vcpu *vcpu, unsigned int size,
>  static int kvm_sev_es_ins(struct kvm_vcpu *vcpu, unsigned int size,
>  			  unsigned int port);
>  
> -static void advance_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
> +static void advance_sev_es_emulated_ins(struct kvm_vcpu *vcpu, unsigned count, int size)
>  {
> -	unsigned count = vcpu->arch.pio.count;
> -	complete_emulator_pio_in(vcpu, vcpu->arch.sev_pio_data);
>  	vcpu->arch.sev_pio_count -= count;
> -	vcpu->arch.sev_pio_data += count * vcpu->arch.pio.size;
> +	vcpu->arch.sev_pio_data += count * size;
>  }
>  
>  static int complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
>  {
> +	unsigned count = vcpu->arch.pio.count;

Opportunistically use an "unsigned int" if you spin another version?

>  	int size = vcpu->arch.pio.size;
>  	int port = vcpu->arch.pio.port;
>  
> -	advance_sev_es_emulated_ins(vcpu);
> +	complete_emulator_pio_in(vcpu, vcpu->arch.sev_pio_data);
> +	advance_sev_es_emulated_ins(vcpu, count, size);

Eww.  The dependency between vcpu->arch.pio.count and complete_emulator_pio_in()
is nasty.  Can you add a comment above count to reduce the likelihood of someone
using vcpu->arch.pio.count directly here instead of making a snapshot?

	/*
	 * Snapshot the count before completing userspace I/O, which will
	 * consume the userspace data and thus clear vcpu->arch.pio.count.
	 */
	unsigned int count = vcpu->arch.pio.count;

>  	if (vcpu->arch.sev_pio_count)
>  		return kvm_sev_es_ins(vcpu, size, port);
>  	return 1;
> @@ -13089,11 +13082,11 @@ static int kvm_sev_es_ins(struct kvm_vcpu *vcpu, unsigned int size,
>  	for (;;) {
>  		unsigned int count =
>  			min_t(unsigned int, PAGE_SIZE / size, vcpu->arch.sev_pio_count);
> -		if (!__emulator_pio_in(vcpu, size, port, count))
> +		if (!__emulator_pio_in(vcpu, size, port, vcpu->arch.sev_pio_data, count))
>  			break;
>  
>  		/* Emulation done by the kernel.  */
> -		advance_sev_es_emulated_ins(vcpu);
> +		advance_sev_es_emulated_ins(vcpu, count, size);
>  		if (!vcpu->arch.sev_pio_count)
>  			return 1;
>  	}
> -- 
> 2.31.1
> 
>