Re: [PATCH 8/8] KVM: SEV-ES: go over the sev_pio_data buffer in multiple passes if needed

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2021-10-13 at 12:56 -0400, Paolo Bonzini wrote:
> The PIO scratch buffer is larger than a single page, and therefore
> it is not possible to copy it in a single step to vcpu->arch/pio_data.
> Bound each call to emulator_pio_in/out to a single page; keep
> track of how many I/O operations are left in vcpu->arch.sev_pio_count,
> so that the operation can be restarted in the complete_userspace_io
> callback.
> 
> For OUT, this means that the previous kvm_sev_es_outs implementation
> becomes an iterator of the loop, and we can consume the sev_pio_data
> buffer before leaving to userspace.
> 
> For IN, instead, consuming the buffer and decreasing sev_pio_count
> is always done in the complete_userspace_io callback, because that
> is when the memcpy is done into sev_pio_data.
> 
> Cc: stable@vger.kernel.org
> Fixes: 7ed9abfe8e9f ("KVM: SVM: Support string IO operations for an SEV-ES guest")
> Reported-by: Felix Wilhelm <fwilhelm@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/x86.c              | 73 +++++++++++++++++++++++++--------
>  2 files changed, 57 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6bed6c416c6c..5a0298aa56ba 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -703,6 +703,7 @@ struct kvm_vcpu_arch {
>  	struct kvm_pio_request pio;
>  	void *pio_data;
>  	void *sev_pio_data;
> +	unsigned sev_pio_count;
>  
>  	u8 event_exit_inst_len;
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a485e185ad00..09c1e64495d3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12378,38 +12378,76 @@ int kvm_sev_es_mmio_read(struct kvm_vcpu *vcpu, gpa_t gpa, unsigned int bytes,
>  EXPORT_SYMBOL_GPL(kvm_sev_es_mmio_read);
>  
>  static int kvm_sev_es_outs(struct kvm_vcpu *vcpu, unsigned int size,
> -			   unsigned int port, unsigned int count)
> +			   unsigned int port);
> +
> +static int complete_sev_es_emulated_outs(struct kvm_vcpu *vcpu)
>  {
> -	int ret = emulator_pio_out(vcpu, size, port,
> -				   vcpu->arch.sev_pio_data, count);
> +	vcpu->arch.pio.count = 0;
> +	if (vcpu->arch.sev_pio_count)
> +		return kvm_sev_es_outs(vcpu,
> +				       vcpu->arch.pio.size,
> +				       vcpu->arch.pio.port);
> +	return 1;
> +}
> +
> +static int kvm_sev_es_outs(struct kvm_vcpu *vcpu, unsigned int size,
> +			   unsigned int port)
> +{
> +	for (;;) {
> +		unsigned int count =
> +			min_t(unsigned int, PAGE_SIZE / size, vcpu->arch.sev_pio_count);
> +		int ret = emulator_pio_out(vcpu, size, port, vcpu->arch.sev_pio_data, count);
> +
> +		/* memcpy done already by emulator_pio_out.  */
> +		vcpu->arch.sev_pio_count -= count;
> +		vcpu->arch.sev_pio_data += count * vcpu->arch.pio.size;
> +		if (!ret)
> +			break;
>  
> -	if (ret) {
>  		/* Emulation done by the kernel.  */
> -		return ret;
> +		vcpu->arch.pio.count = 0;
> +		if (!vcpu->arch.sev_pio_count)
> +			return 1;
>  	}
>  
> -	vcpu->arch.pio.count = 0;
> +	vcpu->arch.complete_userspace_io = complete_sev_es_emulated_outs;
>  	return 0;
>  }
>  
> -static int complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
> +static int kvm_sev_es_ins(struct kvm_vcpu *vcpu, unsigned int size,
> +			  unsigned int port);
> +
> +static void __complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
>  {
> -	memcpy(vcpu->arch.sev_pio_data, vcpu->arch.pio_data,
> -	       vcpu->arch.pio.count * vcpu->arch.pio.size);
> -	vcpu->arch.pio.count = 0;
> +	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;
> +}
>  
> +static int complete_sev_es_emulated_ins(struct kvm_vcpu *vcpu)
> +{
> +	__complete_sev_es_emulated_ins(vcpu);
> +	if (vcpu->arch.sev_pio_count)
> +		return kvm_sev_es_ins(vcpu,
> +				      vcpu->arch.pio.size,
> +				      vcpu->arch.pio.port);
>  	return 1;
>  }
>  
>  static int kvm_sev_es_ins(struct kvm_vcpu *vcpu, unsigned int size,
> -			  unsigned int port, unsigned int count)
> +			  unsigned int port)
>  {
> -	int ret = emulator_pio_in(vcpu, size, port,
> -				  vcpu->arch.sev_pio_data, count);
> +	for (;;) {
> +		unsigned int count =
> +			min_t(unsigned int, PAGE_SIZE / size, vcpu->arch.sev_pio_count);
> +		if (!__emulator_pio_in(vcpu, size, port, count))
> +			break;
>  
> -	if (ret) {
>  		/* Emulation done by the kernel.  */
> -		return ret;
> +		__complete_sev_es_emulated_ins(vcpu);
> +		if (!vcpu->arch.sev_pio_count)
> +			return 1;
>  	}
>  
>  	vcpu->arch.complete_userspace_io = complete_sev_es_emulated_ins;
> @@ -12421,8 +12459,9 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>  			 int in)
>  {
>  	vcpu->arch.sev_pio_data = data;
> -	return in ? kvm_sev_es_ins(vcpu, size, port, count)
> -		  : kvm_sev_es_outs(vcpu, size, port, count);
> +	vcpu->arch.sev_pio_count = count;
> +	return in ? kvm_sev_es_ins(vcpu, size, port)
> +		  : kvm_sev_es_outs(vcpu, size, port);
>  }
>  EXPORT_SYMBOL_GPL(kvm_sev_es_string_io);
>  

I might have missed something, but it looks OK to me.
i mostly checked how the code looks after applying this patch.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Note that I haven't run tested the patches as I don't currently test SEV-ES.

Best regards,
	Maxim Levitsky