Subject: Re: [PATCH] KVM: Add Kconfig option to signal cross-endian guests

From: Laurent Vivier <[email-masked]>
Date: Thu, 09 Jul 2015 11:48:14 +0200

On 09/07/2015 09:49, Thomas Huth wrote:
> The option for supporting cross-endianness legacy guests in
> the vhost and tun code should only be available on systems
> that support cross-endian guests.

I'm sure I misunderstand something, but what happens if we use QEMU with
TCG instead of KVM, i.e. a big endian powerpc kernel guest on x86_64
little endian host ?
Do you forbid the use of vhost in this case ?

> Signed-off-by: Thomas Huth <[email-masked]>
> ---
>  arch/arm/kvm/Kconfig     | 1 +
>  arch/arm64/kvm/Kconfig   | 1 +
>  arch/powerpc/kvm/Kconfig | 1 +
>  drivers/net/Kconfig      | 1 +
>  drivers/vhost/Kconfig    | 1 +
>  virt/kvm/Kconfig         | 3 +++
>  6 files changed, 8 insertions(+)
> 
> diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
> index bfb915d..9d8f363 100644
> --- a/arch/arm/kvm/Kconfig
> +++ b/arch/arm/kvm/Kconfig
> @@ -31,6 +31,7 @@ config KVM
>  	select KVM_VFIO
>  	select HAVE_KVM_EVENTFD
>  	select HAVE_KVM_IRQFD
> +	select KVM_CROSS_ENDIAN_GUESTS
>  	depends on ARM_VIRT_EXT && ARM_LPAE && ARM_ARCH_TIMER
>  	---help---
>  	  Support hosting virtualized guest machines.
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index bfffe8f..9af39fe 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -31,6 +31,7 @@ config KVM
>  	select KVM_VFIO
>  	select HAVE_KVM_EVENTFD
>  	select HAVE_KVM_IRQFD
> +	select KVM_CROSS_ENDIAN_GUESTS
>  	---help---
>  	  Support hosting virtualized guest machines.
>  
> diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
> index 3caec2c..e028710 100644
> --- a/arch/powerpc/kvm/Kconfig
> +++ b/arch/powerpc/kvm/Kconfig
> @@ -79,6 +79,7 @@ config KVM_BOOK3S_64_HV
>  	select KVM_BOOK3S_HV_POSSIBLE
>  	select MMU_NOTIFIER
>  	select CMA
> +	select KVM_CROSS_ENDIAN_GUESTS
>  	---help---
>  	  Support running unmodified book3s_64 guest kernels in
>  	  virtual machines on POWER7 and PPC970 processors that have
> diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
> index c18f9e6..0c4ce47 100644
> --- a/drivers/net/Kconfig
> +++ b/drivers/net/Kconfig
> @@ -261,6 +261,7 @@ config TUN
>  config TUN_VNET_CROSS_LE
>  	bool "Support for cross-endian vnet headers on little-endian kernels"
>  	default n
> +	depends on KVM_CROSS_ENDIAN_GUESTS
>  	---help---
>  	  This option allows TUN/TAP and MACVTAP device drivers in a
>  	  little-endian kernel to parse vnet headers that come from a
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 533eaf0..4d8ae6b 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -35,6 +35,7 @@ config VHOST
>  
>  config VHOST_CROSS_ENDIAN_LEGACY
>  	bool "Cross-endian support for vhost"
> +	depends on KVM_CROSS_ENDIAN_GUESTS
>  	default n
>  	---help---
>  	  This option allows vhost to support guests with a different byte
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index e2c876d..cc7b28a 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -47,3 +47,6 @@ config KVM_GENERIC_DIRTYLOG_READ_PROTECT
>  config KVM_COMPAT
>         def_bool y
>         depends on COMPAT && !S390
> +
> +config KVM_CROSS_ENDIAN_GUESTS
> +       bool
>